i vote for the 10 g too!
although, i would be more worried about getting a heater than getting cartridges for the filter. they are kind of expensive (my marineland heater for my 10g was around 23 bucks) but they're totally worth it.
start a change jar :] I know at least i waste a lot of money by losing coins and single bills. just put singles and coins in it... within a few weeks you should have enough for a heater and prolly some cartridges!