I've owned several SR-JV80 boards over the years, and I would definitely recommend Session as a good all-rounder. As others have commented, the piano patch is a big improvement (up until recently I was using an XP-30 for improv theatre, and I would use the Session piano patch for 80% of the shows â it was perfectly adequate in that context!) As already mentioned, it's not up there with today's piano patches, but certainly usable IMO. There are some really cool other sounds (especially pads) that I really enjoyed on the Session board.
As for Bass & Drums, going by the patches in SRX-07 Ultimate Keys, the bass/drums are indeed a huge improvement. Having said that, if I only had one slot to fill and had to choose between those two, I would go for Session (because of the piano, but it also adds a range of useful sound in other categories).