iTunes is very cheap but the files are also at a lower bit rate. (often 128kbs instead of 320kbps).
Also because they are DRM protected it can be difficult to move them around.
For example I use Traktor Scratch and it wont play tunes brought from iTunes as this counts as copyright even though it is on the same computer.
Beatport or TrackItDown, standard. Beatport has a bigger back catalogue and gets more exclusive tracks, so I go with Beatport. TID is good too though, if they're on both I don't care which I buy from.
DNBA downloads also gives you the option to download the song up to 5 times, which is really good. If you have multiple computers or ever lose a track, it's very handy.
Itunes is garbage for dnb. The quality is crap and not 320, nobody wants that DRM shit, and the selection is shit. Most labels aren't on there. Itunes for dnb is like only buying your dnb selections from Walmart.