I've had a WM-18 for about 6 years. Good mill. To be honest I haven't really done the usual model engineering things with it, more automotive repairs I guess, but it has been perfectly capable.
Sure, there are one or two areas where it could be better, but where it needs to be accurate, it is.
I was erring between that and the 16, but the extra heft, bulk, rigidity of the 18 is so worth it.
I don't think you'll regret it!