The SLO is one of those amps that everyone who is an amp whore (most of us) need to have at one point. I've had two, and have let them go pretty easily after having them for awhile. I know they are some guys' grail amps, but generally, I think if you LOVE a 5150, the SLO isn't going to do it. If you like to switch channels the SLO isn't going to do it. If you like to use the FX loop (and not some type of line out deal to another amp for FX) its not going to do it.
The 5150/SLO thing is an urban legend IMO. They both are 6L6 based amps with a clean/crunch channel that is really neither along with a lead channel, but that's about where it ends.
Like I said though, you can't write an SLO (uh, "an SLO" "a SLO" I don't know) off until you try it. But its not going to be a modern, super heavy sounding distortion as the 5150 tends to sound like.
I did get quite a few chances to play my SLOs loud and on stage, they were fantastic, but I do like other high end amps more for basically everything (crunch, lead, tightness, low end whatever...).
So you'll get one, and for awhile you'll say that you love it and that its the best, but after awhile you will likely sell/trade it to someone who hasn't tried one yet.