I'd go with new, if only because it's a completely fresh start. People who weren't interested by the first comic initially may feel more okay with reading the new one first if it's it's own thing, and then if they really like it they can go back and read the first one later.
Also starting a completely new series will make it easier for the comic to get into trending/popular, and you'll get rid of all of those inactive readers that you have too.