Should I disable wifi if I use ethernet? Is there any benefit in disabling wifi on my Mac Pro (connected to a time capsule via ethernet)?

I notice that with it disabled airdrop doesn't work and iTunes wireless sync misbehaves.
but will it affect speed of LAN transfers?  

Well, you can choose which one would be preferred (by dragging it to the top in the Network preference panel): 



As far as I know you cannot get a FASTER transfer by using both at once...