I have SKS P55s on my hybrid (700C x 38). Great quality, not too bad to fit (take the wheels off - I'm sure that's obvious, but it wasn't to me, and I had a good couple of hours swearing before I twigged). I had to buy longer bolts for the front eyelets, but I suspect that might be down to my frame. I hear good things of the Zefal ones too, no experience of Tor-Tec, I'm afraid.
I've had SKS on a couple of bikes for a few years without any problems.
Just fitted some Tor Tecs to my commuter (because that's what the LBS had) They’re a bit heavier than the SKS, no bad thing, they also have reflective bands on the sides, a red reflector and mudflaps (the link only says front mudflap, mine also has a rear). The quality seems comparable, they're about the same price, I'd stick with SKS for my Audax bike, for my commuter the Tor Tecs are perfect. http://www.zyro.co.uk/product_detail.asp?pid=1469
Elmer, measured across the top (i.e. without bending the tape to follow the curve of the 'guard) they're about 50mm, I reckon. Following the curve, they're more than 60mm, so I don't think the 55 in the designation has any significance really. As User says, Wiggle, or SKS' website will tell you which size goes with which tyres.
I have Tor Tecs on my GIANT SCR4, which is my winter bike, as P.H points out, the Tor Tecs benefit from the light reflective strip on the outside edge of the 'guards, which enhances your visibility, plus the reflector on the rear guard and the mud flaps on both front and rear which help to deflect a lot of the road crap away. And I only paid £8.00 for mine as they came fitted to the bike as part of the deal I got from LBS