While these aren't too common for us Yanks how do they compare to the others?

There isn't really a 7.62x51 Sig 550. There were pictures of one on the internet recently, but this is certainly not a regular production item. The closest you can come would be the Sig 540 which is the 550's predecesor. They are completely different weapons with the 550 being a much more refined design. 7.62x51 Sig 540's are fairly obscure weapons being sold primarily to obscure countries in the far out corners of the world. IIRC, the largest user of the 540 is Chile and they still make them, though poorly. I have fired a Chilean 540 in 7.62 NATO and it was rather unremarkable. The fit and finish was quite poor, but Swiss examples are usually much better. This weapon is very similar in operation and performance to a 7.62x51 Galil, which also happens to be very popular in South America. As far as battle rifles go, I put both on the bottom of the list, well behind the "big 3". The FAL, especially when made by DSA, is the supreme battle rifle with the HK G3 coming in at a close second place and the M14 bringing up the rear. The best thing about the Sig 540 is its "mystic"....if it were common and availible, it wouldn't be held in such high reguard.
Personally, I would opt for the 7.62x51 Galil ARM.... A 7.62x51 Sig 550 is another story, this would be an amazing weapon. The equal or superior of any autoloading 7.62x51 assault weapon.

Ryno is pretty much correct. I had a Sig AMT which is the civilian version of the Pe57/540. Beautifully made weapon and functioned well, but I would consider it a collectors oddity, not a main battle rifle. The 550 series is in the assault rifle category and the export versions are 5.56. Everyone wants one because, well you couldnt get them until (hopefully ) now. Ive shot them and they are good rifles but have alot of company. I prefer the AUG by far over the 550 and my FNC by a slim margin.