My back feels really good when closer to parallel with the ground and stretched out quite a bit but my neck kills in that position. Would be OK if I didn't have to look where I was going.

Bolt upright position kills my back. Every bump compresses my spine and I find that extremely uncomfortable.

There's a happy medium somewhere in between. It'll be different for everybody.

I'd totally go for a nice rigid MTB if I was 300 lbs, though. I used to have more belly and I know that a low position is impossible with said belly.

Hell I have 3 of them as it is. I try to look for light, cheap ones. KHS Comp is 25 lbs, Mongoose IBOC Zero-G is down to about 23 lbs, ParkPre Pro 825 is about 24 lbs.