Both units do what it says on the tin. For simple triggering, the Yongnuos are (generally) cheaper and as reliable as the Cactus units. But there are differences between them that may be important, depending on how you plan to use them. I use the V5s because of some of the additional features they offer.
The Cactus V5 has an external rotating channel selector; the Yongnuo RF-603 uses DIP switches inside the battery compartment. Not only does that mean you can't work with multiple lighting setups simultaneously and independently (which is often the case when shooting sports if you can "wire" the arena), but it also means tearing everything down to troubleshoot firing problems because you can't just look at the settings when everything's set up.
The Cactus V5 has the ability to work with groups of flashes separately or together. For complex lighting setups, that means you can test each of your lights individually (on channels 2 to 5), then set the transmitter to channel 1 and fire them all.
The Cactus V5 has a 1/4"-20 thread for mounting (although it's in an awkward position, so you may need a standoff or a double spigot to use it with some swivels so that the hot shoe doesn't interfere).
The Cactus units have a locking hot shoe. It's a screw-down lock rather than a lever type, and it's in a tight space that may make tightening and loosening difficult, but at least it's there. The Yongnuos just use a friction fit, so it's possible that they may go astray at extreme angles if your stand's cold shoe doesn't have a clamp.
The Cactus V5s will sync at up to 1/1000 of a second; the Yongnuos max out at 1/320. That's only important if you have a camera with a leaf shutter or an electronic shutter (compacts, some early DSLRs, particularly the Nikon D1, D70 & D50, and medium format -- but if you're shooting MF you can probably afford the name brand stuff).
The shape of the Cactus units is slightly more likely to cause mechanical interference when trying to mount them to some kinds of light modifiers using the hot shoe. I had to modify my old-pattern Lastolight EzyBox brackets, for instance. It was about two bucks in parts from the local hardware store to effect the fix, but it's something you need to consider.
Again, those things may or may not matter to you. The extra features on the Cactus V5 cost extra money, but even though not everything is implemented as well as I would have liked (and I'd like to give the case designer a bit of a talking-to), I decided to go for the V5s, since I know I'll be doing stupid photographer tricks that would be more difficult with the groupless Yongnuos.
