Inline chillers are a bit more efficient than what drop-ins are, but it's not a big difference. Having said that you have the added pains of plumbing and the pump to run an in-line. Going with a 1/4hp on that size system will be more than enough for you. My biggest suggestion is just to make sure that the chiller is properly vented in the front and the back so that it's getting good air flow, and not blowing all of the heat that it's removing from the tank right back into your stand.
