I've been running a pair of LSA-1 Signatures in conjunction with a Salk Songsub for the last two years and am very satisfied with these speakers. A few weeks ago a friend brought over his Sierra 1s and we listened for about 5 hours. Music ranged from Norah Jones to Fiona Apple to Creed to Bach to Pink Floyd to BB King to the Bottle Rockets to Ray LaMontagne to Tom Waits to Clapton to The Cramps to JJ Cale to Cowboy Junkies to Louis Armstrong to Jeff Beck to Mark Knopflerin other words, a bit of everything.
My take was that the Sierra 1 is a little darker sounding/laid back than the LSA 1 Sigs. Run full range the bass goes deeper on the Sierra, but is muddier/looser in my room. High-passing at 50Hz (where I run my LSAs) this tightened up (at volume you could see the Sierra 5.25" mid/woofer working it's butt off, and the excursion was not as linear as the LSA 6.5"), but I still found the LSA bass to be tighter.
Mids on both are liquid and detailed, conveying ambiance and inner detail very well, with excellent tonal acccuracy and timbre. Imaging was very good to excellent with both; pretty much a wash. Same with soundstage (disclaimer: my room isn't the best for either, but maybe not the worst).
Highs were a bit more forward on the LSA, at the cost of becoming a little harsh on lesser quality recordingsif you listen to a lot of older stuff and hard rock, go with the LSA-1 Standards or Sierra 1 Standards (I suspect that NrT tweeter will be more similar to the LSA, if not even more forward and revealing); I know for a fact that the LSA-1 Standard is a lot more forgiving of bad recordings, and suspect it is the same with the two models of Sierra-1.
Price for me was also a wash. I got the LSA Signatures for about $50 more than the Standards go for. I ended up with the LSA for the simple reason that I was able to audition the standards (they stomped a pair of Paradigm Studio 20's into the ground, at least as far as tonal accuracy and imagingsame room, same gear, same music).
I could be happy with either paired with good sub(s)in my 13X16 room, and feel that any shortcomings of either, at least in the treble and midrange, probably has more to do with my digital front-end than with the speakers.
Working as I type to correct that. ;)
Hope this helps.
SR