As with audio gear, I think it's important to upgrade photo gear for specific reasons -- not just to have newest-latest-greatest. Since all the cameras in question are APS-C, the benefits of full-frame aren't at issue. A few questions:
- Are there particular image characteristics you're not achieving in your current photos that the 7D or 50D will provide (or image characteristics provided by your 350 that you'd like to get ride of)?
- Are there specific features in the 7D or 50D that will help you achieve better photos or improve usability significantly?
- Are there shots you're trying to take today, but are missing outright due to the 350 feature set, and that the 7D or 50D will allow you to capture?
If I had both a 24-70 and 70-200, my biggest issue would be the last bullet -- due to APS-C crop factor and/or lens options resulting in a lack of wide angle capability. Full-frame isn't an option for me (Pentax doesn't currently offer a FF body), and it would be outside of my budget, anyway. Which leaves lenses. So for me, in your shoes, I'd probably re-purpose the body budget for glass -- specifically a wide or ultrawide lens.
I don't find the field of view of 24-70 wide enough on APS-C (equivalent to ~39 - 114 FOV in full frame in Canon-land). I'd pick up something wider in the 12-35 range (I'm unfamiliar with Canon's specific lens offerings). In Pentax-land, while lately I've been on a HQ-but-inexpensive legacy prime kick, the ultra-wide primes exceed my prime budget, so that probably means a 12-24mm f/4 will join my lens stable in the near future. $0.02
(On a side, but related, note: to an even bigger degree than in audio, my biggest obstacle to recording fantastic images isn't the quality of my gear -- it's my own lack of creativity and an untalented, undeveloped mind's eye for composition.
No gear upgrade will fix that problem!)