It is a very tough decision. Are you planning on just a two channel set-up, or gearing more towards a home theater rig? The general consesus is floorstanders for two channel and bookshelves for multi channel. Again, that is generally speaking.
There are pluses and minuses to both. As Tom stated, the need to buy stands for bookshelf speakers lessens the gap between floorstanders. They are generally easier to place than floorstanders--not because of physical size (when placed on stands they use the same amount of space), but their lower bass output makes them less susceptible to ugly room nodes.
I used to run bookshelves in a two channel set-up by themselves--no sub. I tried both and found I liked the sound better without it. That was after a lot of trial and error and trying different crossover points. I found the sound to be a bit smoother through the midrange. Luckily, the bookshelves played pretty well down to about 40Hz. I'm not saying it's impossible to integrate a sub into a two channel system, just difficult for me in my set-up. There are many on the boards who wouldn't think of listening without a sub. Nothing wrong with either way. It's all about what you like, not what others say is best.
For home theater use, though, I think a sub is a must. And it will work equally well with a bookshelf or a floorstanding speaker. I use bookshelves in my HT set-up, and still get great impact with a sub. I'm not so concerned about fluidity through the crossover point as much, just so there is not a gap.
XLcomm, hopefully more will respond and offer more viewpoints. It's tough finding a combination that works. It took me a few years of tinkering and trying many different things to find out what I like and dislike. And I'm not even close to being done!
One thing is good--you're looking at Axiom speakers which excell at whatever you want to throw at them.
