the batt box/preamp combo has subtle advantage over just a batt box. you'll be able to make a quality recording with either, so if you can afford the preamp option you might as well, just remember it is not absolutely necessary.
a seperate preamp, like an ad20 or a ua-5, is a much more noticeable difference. it looks like you'd have to save some more for those, you can upgrade later if you like.