Its purely preference. I use the 10 x 8 combination in all my builds. I like the richness that the 2 speakers imparts. Rich and a bit more complex, but not so much that it overshadows the amp. LOVE that part of the design.
There are a lot of reasons one might just use a 12. You can get a more standard cabinet from Mojo or something that way. But I think you will find many on the forum here extolling the virtues of the 10 x 8.