Easiest to use is probably the 420EX since there's no control whatsoever to fiddle with (except for the FP switch); however:
1. The 420EX is less powerful than the 550EX (Guide Number of 42 vs 55 in meter).
2. The 420EX is an automatic mode only flash whereas the 550EX can be manually overiden.
3. In the wireless mode the 420EX can only act as slave units whereas the 550EX can also be programmed as master (ie main controller from the camera).
4. There's no high-frequency flash control on the 420EX, hence no stroboscopic action (also serve as modeling lamp function) or built-in wide panel for wider angle/catch light as on the 550EX.
Not all is lost though you can still get a Sigma 500DG Super
which is a virtual clone of the 550EX for less than the 420EX (and use the 420EX as a wireless TTL slave)!