You found a new, complete Spike's somewhere? They are back ordered for the next year or so. Nice find and a great AR.
I'd go with the Spike's without thinking twice. Nothing wrong with the Sig either. Also, it looks like you found one with a mid length gas system. I have a carbine (M&P) and a middie (Spike's upper) and it combined with the Spike's ST-T2 buffer and Full Auto carrier make it one of the smoothest AR's I've shot. It also has the Dynacomp brake on it and that also helps. Spike's service is outstanding and they really stand by their products.