I've used both of these and it's like comparing apples to oranges when you want to make an apple pie; they're both fruit but only one is actually supposed to be used in the pie.
eJamming is a business that's been around for a while. When it first started you could only use midi. Then they added audio, but the latency was horrible. So they came up with a system that worked "with" the lag, by having each musician play however many measures behind each other, like learjeff was saying. Forget about trying to count off "1,2,3,4,play...", it wasn't happening.
Flash forward to the present day and they recently signed up with Fender and supposedly created new software that minimizes or gets rid of the latency. This involves opening your computer ports to the eJamming software so you're on a peer-to-peer network, like using torrents or limewire to download stuff. That makes it more involved getting set up and that only adds to the problems of their already buggy software program. The program itself is pretty good, you can create your own tracks and record takes together, etc. However, it does require ASIO drivers to work and this usually means having either an interface or getting ASIO4ALL to work, which can be iffy for some. Some people have no issues, while some can't get it to work at all.
Once and IF you get it to work, I will say it seemed to make the lag a lot better than it used to be, but that still also depends on everyone's internet speed and their computers. They say you need to have like 2.0mb upload speed in order to play well with more than 3 people. I've seen some of their vids that shows 3 guys playing "in time" and that must've been with absolute perfect conditions all around. I still ran into lag and I still don't think you can count off or play covers and such, but as the saying goes,"your mileage may vary",lol. However,it does work pretty well to jamming out improv and stuff. But that's really just going back to the idea of coming in a measure behind the person you're playing with. And that brings me to the next problem for most: Who to play with.
Every time I have ever tried the eJamming 30 day trial, over the past couple of years now, there simply aren't enough people using it. Unless you and some friends plan to log on at the same time to play, there needs to be a butt load more people in the pool in order to find someone else to play with. I thought this would be solved once they signed with Fender, but apparently Fender decided not to promote it like they could have. The result? There never seems to be more than 20 people on at once and that seems to be at peak. It usually averaged about 10 people. And it's not as easy as we'd like to think to just start jamming with someone. There are a lot of variables that go into finding someone you can play with like that, least of all that there are only 10 people in the room to try and make it happen. Especially if 9 are shredders and the other one plays country, lol. The last time I tried eJamming was this past January or so and nothing much had changed, although they're supposed to be coming out with new stuff.
It costs like $10 a month to join and at this time, for me, I don't think it's worth it. If you have people or band members you specifically want to log in and play with though, it may work for you. Good thing there's a free trial.
Online Jam Sessions came out a little over 2 years ago and is run by a single individual out of his home in Nevada. Unlike eJamming, it's not complicated at all to set up and get started playing and requires no software to download. It's as easy as turning on your webcam and/or plugging into your computer's audio line-in. That's because it's a video chat room that uses Adobe flash. This is the same flash that a lot of video chat sites use, like www.tinychat.com
where you can make your own free rooms. Why, one could argue you could just as easily use tinychat instead! LOL, and you'd be right. The software that online jam sessions uses is a free community software. You can find, and use, the exact same video chat room at the software developer's own community site that's free to join, http://www.boonex.us.
And there are also other free ones with the same video chat room.
Much like eJamming, there is latency. Ok, a lot more latency than eJamming has, especially now. And the audio on eJamming is better. But you'd expect that from a site that's using a video chat room as the "jam room". It's a chat room and was intended for people to chat and video conference, not play music together. But like eJamming, this can work and be fun for jamming. And it can be a pretty cool place to hang out and chat, show and trade licks with the video and even have some cool jams. But I've seen a lot of people come in, like band members who want a way to practice online, and they soon leave disappointed because it doesn't work. It would be fine for working on ideas and what not, but when it comes to actually playing together it's pointless. You'd have better luck with eJamming. Also like eJamming, online jam sessions suffers from a lack of people. OJS doesn't do any promoting, relying more on word of mouth, myspace and the like. As long as it's free to advertise,lol. The home page may boast thousands of members, but you'll only find about 10-15 at the busiest of times. Usually it's the same 5-8 regulars who have been there the past year or so, most of whom are simply given "free" time to hang out and entice new people to become a paying member. And that doesn't happen very often, if at all. And then there's the owner...which I won't even go into (don't mention Skype in his jam room,lol).
When I first found OJS it was $10 a month or $100 a year. Then he ran a "special", for almost a year, at $5 a month or $50 a year. Now that's the current regular price and I think he has a $150 lifetime "special". They also have a "free" 20 or 30 day trial. The site hasn't grown at all in over a year and given it's a video chat room you're paying for, I can't justify even $5 a month. Tell them I said 'HI!' if you do check it out though,lol.
If your intention is to play online and you can only choose between OJS and eJamming, I'd spend the extra $5 and use eJamming. Fortunately, those aren't the only ways.
The best free alternative to either, and for a while now, is Ninjam. Ninjam, at www.ninjam.com
, is totally free and runs as either a standalone program or as a plugin that comes pre-installed with the Reaper recording software. Of course, it can also have latency depending on the users, but it's free. Also, there's only room for 6 people in any one of the 5 or so ninjam servers, but it's rare to find all of them full. Like both OJS and eJamming, there's a low number of users. At least Ninjam has an excuse though,lol. It's free and therefore there's zero promotion. Depending on the computer and equipment, the audio seems to be as good as eJamming. Better than the video chat rooms like OJS uses anyway.
So ultimately, as of now, I'd say Ninjam is the better one,lol. Let me know if you have any other questions about them.