Apple churn out a new model every five minutes, so by the time you've forked out for one, the "latest" version will be grinning at you smugly from the glossy pages of a catalogue within a few months. iPod's are nowhere near as good as they were when they first came out and the battery life is shocking when compared with pretty much any other brand. They break more easily too so if you do get one, be careful not to do anything silly like picking it up or looking at it.
But if you really want one, then get whichever one is most suited to your needs - if you don't need 5G and are never likely to need it, then stick with the 4G. But if the 5G is hardly any different in price, you may as well go the whole hog and get the biggest you can afford.