simple answer is to buy the biggest capacity with highest C rating that you can afford.
prices are getting better all the time so shop around.
I have seen intellect 40c 5000mah selling for around $65.
casterracing EX-1 Pro, SP Silver Arrow
TOP Racing Scythe, SP 17.5, SP GT Pro 2.0
TOP Racing Scythe, SP 4.5 V2, Hobbywing Xerun 120SD 2.0
TOP Racing Scythe, SP 13.5, Hobbywing Xerun 120SD 2.0