The differences between the ubuntu variants can be a bit confusing if you're not familiar at all with linux. Basically they have the same core, but use a different desktop system. This is because linux isn't tied to a single desktop like windows. Other than personal preference the most significant thing for you would probably be that XFCE (Xubuntu) is a little bit lighter on required resources. With that little RAM even XFCE might be a bit slow. There's other even lighter window managers, but you'll need to install them on your own, which isn't too difficult in ubuntu. FLuxbox is a good one that I used for a while. There's even a ubunutu varient
that uses it, though it's not an official variant.