$space would be best as far as the scripter is concerned, the implemention details doesn't matter much, we're asking for the possibility to get our string intact in a scripting language, not the moon! Check out mestinkbad's post, you cannot workaround that problem, it's a serious thing although I guess you also screw people using consecutive/leading/trainling space in their filename
The flexibility is that we could get mIRC to use a mix of the old and the new tokenization method in one line, like I said we more than often want the old/current tokenization.
Quotes would also achieve that flexibility, but it would break backward comp
