I don't think there is any significant difference between them other than the obvious one of size. The 64 gives you a slightly bigger screen, front-mounted buttons and supposedly improved satellite reception (bigger aerial). The last point is a bit of a non-issue I think - as a long-time Etrex user I've rarely encountered reception problems, and almost never in the UK (city centres apart) - and I suspect for the 64 aerial to be fully effective the device needs to be held vertically. For me, the 64 package is too big for handlebar furniture although plenty of cyclists do use them.
The latest 'x' generation of Etrexes (20x, 30x) seem to have very dim low-contrast screens though - I find myself using the backlight a lot on mine, whereas with the older models (20, 30 and the older Vista) I never needed the backlight in daylight. This (screen brightness) may be a point of difference between the current Etrexes and 64s.
Another popular AA-using option is the Oregon range - bigger screen, touch screen, a good GPS but again too big on the bars for my taste.