NYMEX vs CBOT contracts?

Aren't NYMEX ones more liquid (thus tighter spreads)?
Why trade CBOT then?

 

The margins were lower and that attracted traders. However, the volume is so low at this point CBOT is almost untradable for daytrades and scalps.

They also have the mini golds.

 

The CBOT contracts may have much cheaper exchange fees than NYMEX.