The lead channels are very similar and the X88R has more gain on the middle channel. The X88R basically is a Steve Lukather thing to begin with, to keep him from having to cart around three slaved heads - and he likes his crunch channels gainy. So if you need to boost the X88R you need to boost the SLO as well, and if you you´re looking for serious high gain metal rhythm you probably do need to boost. They´re more heavy rock, classic metal, sort of fat old-school vibes.
I like the X99 best myself, sounded a bit bigger than the X88R I also had and has the original SLO crunch channel which I prefer.