Acei - from your second post it sounds like you're building a nice 5er, and you've given this some thought.
We have a recent 1T SRW diesel pulling a 34ft, 14000lb trailer and love how it performs. Make sure you don't go over payload, but Ford/Ram/Chevy all upped capacities in last few years so as long as the truck is new or close to it you should be fine.
For the rig you're planning, I think you'd be happier with a diesel.
One other thought that other's can jump in on, is I really like the 25ft length but wouldn't be afraid to add a foot or two if it gets an arrangement you like better.
Good luck
