I'd seriously go with the Wonder-Wave texture. (I think EF has it in the "Ice" variety.) It's the boyfriend's favorite.
Aside from Wonder Wave, he also enjoys the Vibro, but not the original version.
If you are going to go for anything, I highly recommend against the original. The textured versions feel so much better. We had the original, he hated it, and never wanted to try a Fleshlight again - especially for as much money as they cost. It's just going to feel tight - there's no texture to it.
STU is a pretty big change in intensity compared to the rest. If you are not sensitive, I'd go with STU. If you are *Extremely* sensitive, go with original but be forewarned the lack of texture may make you hate it.
Like another reviewer, I'd highly recommend a Fleshlight Girls product. Ignoring the fact that they are modeled after girls, we one does have a unique texture. The boyfriend has fallen in love with the Tera Patrick Twista lately.