I would recommend the 420EX over any other Speedlites, because
(1) It's a Original Canon. No future compability problems.
(2) It covers all the 7 focusing points of the 300D for focus assist. 550EX only covers the 5 horizontal ones (it's designed for the 45-point focusing of EOS 1-series instead) and 3rd party flashes cover only the center AF point.
(3) It's good enough for most amateurs and can get one 2nd hand for less than $300.