12 AWG has a higher current carrying capacity and lower voltage over a given distance than 14 AWG due to its lager cross sectional area, but the corrsponding increase in diameter can make it difficult to get through the holes in the PCB of some ESC (I had to change the power leads on my Losi Excelorin ESC and used 12 AWG as that was what I had in my pit box and it was a very tight fit to get it through, had to tin the end of the wire very carefully to make sure no strands poked out as they stopped it going through).
My ESC came wired up with 14 AWG wire on both the power leads and motor leads originally and that was certainly enough as the wire never got warm to the touch after a run (that's a sure sign you're carrying too much current!).
I've been franticly searching on-line for a current capacity chart for silicone wire to back up my thoughts as it's been a while since I used my Electrical and Electronics HNC in anger, but I can't find anything from the manufacturers, only suppliers (such as this one
http://www.4-max.co.uk/silicone-wire.htm) but I don't know how accurate it is......