I found this product on the Home Depot website. It looks like it would be better for the floor than standard backer board. Would it be difficult for a novice to install? The only thing that gets me a little confused is the joining the floor to the wall. It looks like they have something that you have to install there and I don't get it.

Ditra is perfect for tiling on a concrete floor. What makes you think it is better than a backer board, are you on concrete? Hardibacker is my personal favorite for most othet sub floors. Your floor is not joined to the wall, they butt up to each other but are not connected, the baseboard covers the seam.

It will be going onto plywood. I just want to be sure I am doing the best I can with the floor, I don't want to take any shortcuts. If I were to use Hardibacker I think my floor would end up being higher than the floor entering into it. The bathroom is on the second floor.

Ditra will likely raise your floor higher than 1/4" Hardibacker. In my opinion Hardibacker will be stronger.