Yeah the X-Wing ships are made from a firmer plastic with slightly better detail and a slightly better paint job (which is to say, the paint jobs on the SWM ships was actually quite good compared to the figures). The ships from
FFG are also larger and scaled appropriately where as the fighters from Wizards of the Coast are all the same size.
That said, you have a much better selection of ships to chose from if you get them from the old Star Wars Miniatures: Starship Battles line and even the rare ships (and I do mean rare. Blind buys.. blech) will probably only cost you the same as a single new X-Wing while the common ships can probably be picked up for £1.50 each. Also if you are interested in different eras, this is your only option unless you can get the ship you want on Shapeways.
So I wouldn't totally count them out, particularly if you aren't after ships
just to play X-Wing with..
The X-Wing is supposed to be an ARC 170. Stupid WotC making ARC's rare... *grumble grumble*
Edit: Ahhhh I should have refreshed before I replied. Yeah if you are after totally different scales for other purposes then there ARE options. I used a few Revell kits in my Star Wars
RPG campaign..