There's no "best" or "better" answer to that question. All things, as Einstein said, are relative. Depends on what you mean by "editing". Depends on what kind of video you're working with and what you're doing with it. Depends on what you mean by "editor". VirtualDub
can be used as an "editor". So can a whole ton of free "editors" and converters, most of which work with XP. Avisynth and Virtualdub
are for repair and restoration -- their filters are 32-bit, so you have no overwhelming speed advantage with Win7/64.
Older motherboards that accept AGP cards for capture are still in use. Older means slower bus speed relative to new machines that can't take AGP cards and are limited regarding capture devices but can use faster CPU's and faster RAM and bus speeds. XP can be installed on both machines, likely Win7 can only be installed on the newer BIOS. So the ifs and buts also depend on what kind of PC, motherboard, CPU, etc., you're taling about. Newer NLE's like Vegas Pro
, Premiere Pro, AfterEffects, and typical push-button junkware from Walmarft etc., work only on Win7 machines. Most NLE's can't do many things that Avisynth and VirtualDub can do, and vice versa.
Short answer: It depends. Each has pros and cons. Having both covers all the bases.