Originally Posted by
Mithrandir
They said that the (A) 540 style pedals are easier to clip into because they're double-sided, and (B) allow you to use recessed-cleat shoes, which makes it easier to walk around in. [Plus]
On the other hand, I like distance riding, and they said that (C) the 105-style pedals have a much larger contact area and will likely be more comfortable on longer rides.
A - That hasn't been my experience, but most people would agree. The pedals are weighted so that the side you need to clip into winds up being where you need it to be 90 % of the time.
B - Shoes without recessed cleats aren't impossible to walk in, just awkward and uncomfortable. You can get cleat covers, but it's kind of a hassle to carry them around, then stop and put them on, before you go get a drink of water. I saw a guy slip and fall walking in a restaurant once, because his shoes had no traction whatsoever, and the floor was smooth tile. I like SPD pedals, and shoes with recessed cleats.
C - This really doesn't matter, if you have stiff shoes. I've done plenty of long rides in SPD pedals (A-520) and carbon soled shoes. It's great!