Both systems along with the Pro system are now very reliable. The 2.4 is, as Alex suggests, the easiest to install, and also the 'cleanest' looking. Additionally, my personal opinion is that the receiver kit (that's got rid of with the 2.4) was always the weakest part of the system. If the additional pricing of the 2.4 compared to the SL is okay for you, i'd recommend the 2.4.
Fearby, give me a shout on the prices, we have the best UK prices!
Cheers
Ric