JJS--yeah, I don't think I'd want the Argosy as a lake boat, it doesn't track well enough.  Great for little rivers and creeks, though.  The Rockstar is pretty fun, I've only paddled it briefly, though, so I can't really review it.  My preference for the Merlin is based mainly on the fact that it's what I paddle and I have got to know it's quirks.  I find it has great secondary stability, and pretty good primary with the foot brace.  It's a brave little boat.
Cheers,
gp