Is the Ally physically larger? Yup. Are planetary gear-sets better? Yup. Does it prove to be better in the real world?
Planetaries are larger, aluminum housing and 4 pinions. Hmmm, I think we pretty much figured out 6 pinions and a steel cage is a must for high TQ applications, either that or managed it correctly.
The clutch packs are 3 and maybe 4 disk. More surface area but if it can't get applied correctly or tight enough it doesn't matter.
The bigger-is-better ideas get tossed when physics comes into play. Its all about leverage and thats what usual kills the trans.
We seldom trash hard parts in a 48RE unless the power levels get really high and it is unmanaged. The 48RE needs 1 or 2 small tweaks to handle 500 HP and they are easy to do. The Ally needs quite a bit more retrofitting to handle the power and not eat itself up. It can be done but it is costly.
As has been posted, real world performance is what counts and thats where simplicity is winning out.