^Honestly, I don't use a lot of extra functions, but I'm pretty sure the 991's do nearly if not everything the 115's can, plus a lot more.
There's just a few things that I had gotten used to, I think. The ergos. And the way in base (dec/hec/bin/oct) mode, my 991 fills all the spaces to the left with zeroes, for god only knows. Especially with binary, it means you sometimes have to scan a few extra spots to the left to make sure you are reading the entire number. It's better left blank, IMO, the way my 115 does it.