Rooms are identical between Cat 5 and Cat 6.
As already stated, Cat 5 is one deck higher, but otherwise the only other difference I've ever noticed is that there are fewer Inside Cabins (therefore fewwer guest) on Deck 7.
If the upgrade is a freebie, go ahead and take it. If it's going to cost you anything, tell the TA "No Thanks".
[Edit]
and be much more convenient to everything
FWIW: Since you are near the aft elevators/stairs, you are already conveniently located to most of the resturants. Midship would place you closer to kids labs, but you'd also be one deck higher from the labs. Being on Deck 7 makes Deck 9 (pools, scoops, etc) a little more accessable (it's just two flights of stairs up), but it also means that everything on Deck 3 is 4 decks below (not bad going down, but a pretty good hike going up). I reference this information because I personally detest waiting on elevators unless absolutly neccessary (for example, I'll walk Deck 3 to 7 rather than wait on elevators, but I'll wait for an elevator going from Deck 1 to 7 when getting back on board from ports of call).
So again, I say that if the "upgrade" is going to cost you, I wouldn't bother. If it's a freebie, it's pretty much a toss-up.