My first "assault style" rifle was a Colt AR15A2 R6600 that I purchased in 1988 (I think). I still have it, of course. You never forget your first....
I purchased a "pre-ban" semi-auto SIG 551 in 2001. I'm not law enforcement or military or a dealer to such, so I had to purchase the rather expensive civilian SIG 551. Very few were imported into the USA prior to the 1989 ban. You don't see those for sale very often here in the USA at (what I think) are attractive prices. I know of only one that's now available, and the price is in excess of $13,000 (no, it's not mine).
Very different rifles. You might want to handle them both and choose the one that feels best to you. You can't go far wrong with either. If I had to abandon all but one of my rifles, I'd choose the SIG 551 to keep. That's about the best testimonial I can give.
(Sorry... Moderator... please don't kick me off the AR15 site... I love my AR's too!)