In all honesty, there is no "real" difference. Yeah, they come with different window managers. So? You can install any window manager you want, and can have as many as you want. I guess it just makes life a little bit easier since the one you prefer is already setup.
As far as performance, Ubuntu is the wrong way to go. Too much bloat these days. KDE and Gnome and Unity are all "high performance" window managers. Use something with LXDE/XFCE/openbox/etc. Look up the wiki on window managers for more choices.
As far as distros, Peppermint one is a great choice. Crunchbang would be another, but it it requires a tad bit more experience with a linux OS then someone who uses Ubuntu and doesn't know the OS well. There's also Lbuntu (or something like that) which is Ubuntu with the LXDE WM i believe. So i understand you have a slow connection, but with many of these distros, there not exactly large anyway. You could easily get them, even on a dial up connection. Dunno. Just throwing some stuff out there.