tightVNC 2.5.X and even pre 2.5 supports multi monitor. When you connect, you get a huge virtual monitor. However, this is also has disadvantages. UltaVNC (Tho when I tried it, was buggy in this area) allows you to connect to one huge virtual monitor or just to 1 screen at a time. (With a button to cycle through them) TightVNC also plan to support such a feature.. (When , no idea) This feature is important as if you have large multi monitors and connecting over a reasonably slow link.. The screen updates are just to slow.. Cutting down to one monitor to focus on is desirable.
I like tightVNC, but UltraVNC seems to have a few more features right now..
I have found tightVNC more solid. And that is why I have stuck with it.
I would try both. They both work well, but I imagine one would suite slightly more then the other.