So I bought a used sphere and the motor wires are to short. I went to my local hobby shop and they only had 12 or 14 awg wire. (sphere calls for 13awg) Is there any benefit to using 12awg?

unless you have an extreme power system then 13 is plenty, you will not notice any difference with 12 gauge. BUt it will not hurt anything if you do switch to get the extra length you need.

What kind of car? The only time I would expect a problem with 12 guage is in a pan car, might bind up the pod movement a little.

It's a T4...

go with the 14

Ya 14 will be plenty and easy to work with.