I own the LSS and spent some time playing the Stiletto Ace Combo.
To me they are very different amps that I would think of as an either/or or get one...then get the other.
To me, the LSS is a wonderful creature that is sort of unto itself. It has a wonderful selection of tones and far less tweaking to get them. I think it is best with single coils, but frankly my LP Custom sounds terrific through it.
The Ace from what I understand is a way to get into the Marshall EL34 sound areana. Sealed cab with more punch than the LSS. There is a more sudden response to your pick attack and I think it really does a good job of kind of getting a more 'in-your-face' sound. For me, I had to spend a fair bit of time throwing switches for channels and clean/OD to get an idea of what sounds were in there. There is a part of me that always wonders, "If someone wants a Marshall sound, why wouldn't they just buy the Marshall?" Having said that, there are many of us who are very loyal to the name and quality of Mesa.
If I could only own one amp that is pretty darn versatile and gets me wonderful sounds, it would be the LSS.
My suspicion is you may well end up with both.