A teamate has the mini and uses quality duracells, gets four or five days out of each one. Don't skimp! My gun will eat a poundshop special inside a few hundred shots, yet give thousands off a decent duracell...
I wouldn't use the spyder battery. It just has a higher voltage output - it won't nescessarily last longer and may fry your board too!
Dito, i have had 4 games out of my mini before replacing the battery, i was using a Durecel Ultra, which are more or less the same as procell.
Danny buy a box of 10 Durecell procell in bulk mate via the internet, it will last you the year.