Playing the SLOs for a while, and spending a lot of time with the E90, they are very different. I prefer the SLO, but the E90 is great. The E90 can get heavier, I don't find it as articulate though. I never found the SLO fatiguing, but cabs have a lot to do with that. I LOVE the crunch channel of the SLO, much better than the E90. I don't hear as much body in the E90, again not as articulate as the SLO. E90 has a better clean in my opinion. E90 is more versatile as stated.
Honestly, either kicks booty. It really depends on what style you play and what you like. Both are major players. I think if I was really going after the E90 vibe I'd check out the new Dar amps as well. Really brutal.