Ben,
Engine and transmission for the B7800 and B2910 are the same. The PTO hp is the same as is 3 pt. lift. They both use the same size implements, so from the standpoint of ability to do a particular task, they are equal.
I have a B2910, and I'll tell you in my opinion the order of importance of the "creature comforts" that the B2910 has that are lacking on the B7800.
1.) The telescopic lower links - makes connection of 3 pt. implements a breeze.
2.) No tool ratchet adjustment of 3 pt. hitch - again, makes adjustments to implements fairly painless.
3.) I think the position control on the B2910 might be a bit better than the "1/4 inching" valve of the B7800, but it might not matter in many applications.
4.) I like the suspension seat - especially after I suffered a back injury.
5.) The cruise control. I don't use it for speed control, but rather, I use it to drive the hydro pedal forward so I can use my right foot to "brake steer". Helpful when using the loader in some applications.
6.) Adjustable steering column - not really a big issue with me, but when the kids sit on the tractor (shut-off, in the garage), I can adjust the wheel so it's more comfortable to them.
Well, that's it from my point of view. Do I think the B2910 is worth the difference? Yes. Mostly for reasons #1 & #2 above.
I will admit, I've seen the B7800 on the dealer's lot, and if I didn't know how much extra "benefit" those extras added (in my opinion), it would be difficult to justify the difference in price.
Hope this helps.
Let us know if you have any other questions.
~Rick