5 acres is getting into B2601 territory. 5 acres is a wee way to drive, the BX is slow. The B has 3 speeds, and in high goes a lot faster. If you're mowing long strips it's a big difference, but also different just getting from a to b. The wheels are quite a bit bigger so it rides better (still not great, but better). I especially say this because you're looking at the BX 26HP. If you want to spend a bit more for 26HP, I think you really want to spend more for a bigger 26HP machine (B2601). The BX 26hp really doesn't do much more than the BX 23hp, whereas the B2601 does quite a bit more.
I personally wouldn't fill the tires, everyone says to, but I think a ballast box is better. Filled tires are weight you can never remove, and ruts on your lawn are a thing.
I had a BX and now have a B. I like the MMM, and on the B the decks are all drive over (no up charge).
If you want a grapple and SSQA, check the remaining capacity on a BX. They don't lift a huge amount to start with, once you subtract the SSQA setup and the grapple, the useful load isn't big. The B is a bit more (still not huge, but I'd guess the useful capacity with a grapple would be close to double).