two 10's move more air than one 12.
The box for two 10's is bigger than that for one 12.
However 10's are generally cheaper than 12's
If trunkspace is your issue and you want to have just one sub
you could get one 12w3
or one 10w6. I would go with the 10w6, great sub, punches real good,
and definately sounds better that the one 12w3.