I'm running a 3.1 setup right now actually.
It came down to budget, and what I liked. I knew I liked the Paradigms, and the Def Techs. I used to run some Monitor 9's before I blew'em a year ago, and I knew the Studio 100's V.4's were a definite imrpovement especially since Paradigm trickled down some goodness from the Signature series. So I went with that. The two fronts and the centre ended up costing me close to $3,400 CAD, so I was fairly pleased. I dropped $3300 on the Velodyne DD-15 as well, because I knew I wanted a dedicated sub and the Paradigm's though great would not provide the umph I needed.
So now I'm quite handily broke, but all in all I'm quite pleased. When I get the money together, I'll invest in the ADP-590 surrounds to match my system equally. In the meantime, the 3.1 is fantastic, and though I don't get true surround sound when I watch movies like Saving Private Ryan I definently feel as though the bullets are flying past my head.
Time is your friend, haste is your enemy. Invest in the best possible initial setup and work your way up to 5.1, it'll be worth the wait.