Pros of RTW: I know how to mod RTW and I have a good six years of experience doing so.
Cons of everything else: I don't.
If you can find an experienced EU3 modder who wants to make such an EU3 mod, be my guest, but the reason I'm able to do this on a reasonable time-scale is that I've built two major total conversions of Rome already and I know the problems I'm likely to face and can deal with them and debug them very efficiently. So basically RTW has to win from a human resources aspect.
Not all the pros are unique to EU3 either; we can expand the map later, I'm just trying to keep the project a sane size, and we can get some province effects and local area recruitment is fairly easy to do and in the plans anyway.