The 'X' stands for Xeon and is generally meant for the server-business rather than desktop-business like the 'Q's.
There are some differences in heat-production and flexibility for coding (if I'm correct).
For TG a Q6700 would be very nice, unless you really would like the have 2 quadcores in one machine, then you should go for 2 xeon-cpu's because dual-motherboards for a Q6700 or Qxxxx isn't available.