Your battery-or-memory problem is impossible to answer as you have it stated right now. The real question is: how much battery on the plain model vs how many songs on the plain model?
I have an ipod with tons of memory, and I've barely added any music since getting it. Mind you, with 30,000 songs on it, I already don't listen to most of it. So, how many songs can you put on yours? If it is enough, then go battery. If you can't, then start to think about the problem. (Unless you intend to use it as a portable HD as well; mine doubles as temporary-backup-during-reinstalls, for example.)