Hi Steven,
You might try the deck builder on the Trex website. It looks like it is the same program, but it has has a nicer interface and lets you create a 3-level deck which I had trouble doing through the diyonline.com version. The Trex version also lets you cut out corners for hot tubs which I didn't see on the others.
What's weird though about the Trex version, is that there is a step where you get to add your hot tub, benches and tables and stuff to your deck design, but when you go to look at your report it's all gone. I don't know if it's a bug or what, but it seemed to be a totally waste of a step.
Trex Deck Bulder Website
Shawn