for the extra's that you get with the 550d body, i would stick with the 500d
then throw that extra few hundred towards better lenses.
the only diff worth nothing
500d 15mp, 550d 18mp. tbh though 15mp is enough as it is, and bigger megaxpixel is like saying you have a higher post count /
sensor is exactly the same on the two of them
exposure compensation
500d -2 to +2, 550d -5 to +5
the 550d does movie at 30fps, as opposed to 20fps on the 500d
if your not using the movie mode, then no biggie