The XP2 and XP3 are great filters. The Fluvals from what I heard are a little tougher setting up and opening, but once you get used to it and it's set up, it's just as good. I haven't heard alot on the Cascade except from in this forum, which sounds like another excellent choice. I'd get another cannister, run it on the cycled tank for about 3 weeks, then put on the new tank. That way you'll be miles ahead, and may not see much of a cycle spike.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006