The difference is small if you use mini.iso (net install) and a minimal desktop version but it can be noticeable depending on the kernel version, packages that

The difference is small if you use mini.iso (net install) and a minimal desktop version but it can be noticeable depending on the kernel version, packages that you add, 32 or 64 bit capability, available ram, other hw components etc. Debian is faster though, and free from the usual ubuntu bloat, install backports, correct drivers and put i3wm or awesome on it and you'll get a usable machine even if it's very old. Not sure if it would play nice with mac hardware but you could always try.