Tom,
They both work well on your ELAN II. This camera has E-ttl exposition for
the flash (it it�s infact the first to introduce this system) so you need a
new generation flash for get advantage of all the features.
The 420EX is smaller, less powerfull and used with a Eos3 or Eos33 cannot
work as a master in a wireless configuration.
The 550 EX is bigger, more powerfull, more expensive and can be a master
unit.
There is also a 380Ex unit which came out together with the Elan II: also
E-TTL and should cost less, no wireless, less power.
Giampi
Italy
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