I have another back surgery planned for early Oct to repair herniated dics and stabilize spine. I was a big road biker, but I know I can't do that again. I have been riding a friends mtn bike and enjoy it alot. I want to get a good mtn bike with the right geometry for me. Will a shorter top tube help by preventing me from being too stretched out? I hate to suggest this but even a womens bike geometry seems the right thing.
Non Tribuo Anus Rodentum and off to the next adventure (RIP)
Ride some different frame and find out what feels right for you. I've had a disc repaired at L4-5 years ago. Personally I like to stretch out a bit with my torso weight supported a bit by my arms to take some of the weight load off of my back.
I prefer a flat or riser bar that's about 2-3 inches above the back of my saddle as opposed to the full tuck of roadie drops.
Had discectomy last summer (l4-5), and after about 5-6 weeks I was riding mtb, road & hybrid bikes. Had to take it easy, but no issues whatsoever. After 6 months I was snowboarding. This season I don't even think about it.
For mtb, I'd certainly go full suspension. It's just better even if your back is fine. Skip the WSD and just get a smaller size if necessary. Stay away from the brands that set up their bikes with longer top tubes, or just get the next size down. Also stay away from agressive XC racing geometry, like the Epic. Just not comfortable for long rides (similar to racing road bikes). I found that the Stumpjumper FSR is exceedingly comfortable for long rides. My stance is more laid back and upright.
Are you talking about buying a mountain bike for mountain biking (i.e., riding singletrack), or for getting a more comfortable position on the road? I ride with a friend with some herniated disk problems, and even his FS bike bothers him on rocky descents/drop-offs. If you're looking for a more comfortable road position, you really should test out some recumbent or semi-recumbent bikes -- apparently some people find amazing relief from them. My Dad is riding a semi-recumbent trike after his knee surgery, and loves it.