Steam is my favorite DD provider, with obvious advantages like automatic patching, no disk required in the drive, and easy transfer of your steam gaming portfolio to multiple systems (if activation policies allow for it *puke*).
Sometimes they do have good deals but this is mostly for new(er) games and combination offerings. As a DD provider Stean does nothave stock issues (like some of the retailers (on- and offline) have when they want to clear their stocks) so finding extreme deals like the $4.90 mentioned in this topic is very very rare.
If the advantages I listed above are appealing to you then these might warrant spending a couple of more $ for the game but make sure there is no activation policy associated with the game (as far as I know there is no activation policy coming with X3).
I seriously wonder why Steam (Valve) accepts publishers to extend their activation policies through Steam because it seriously affects the Steam experience in a negative way. As far as I know BioShock is the only game wiht an activation policy attached to it currently available on Steam but I did not do a large study on the subject, so if anyone knows others, please let me know. I try to stay away from activation as much as possible and will only purchase games with it, when I can foresee it will not be an issue with me (not too much replay value, fitting in with my estimated upgrade planning).