I didn't realize there was a thread about the two here for five years. But then again, I never searched either. Since this thread has come to the forefront, I might as well add my thoughts.
When doing my research, I too had considered a Wells step. Besides the price, what I didn't like is that the Wells step tilts backward, so shifting more weight from the feet to the butt. Some of the reviews I read also indicated that moisture eventually takes its toll on the Wells step's block of wood, and that the chrome starts to flake off eventually for some people. So, that was that for the Welles step.
I have to say I settled on the squatty potty because they did several things right. First, they provide multiple models at two different heights, with the cheapest one starting at about $20 as has been pointed out. Second, the way the squatty potty is designed, it can be used as either a step stool, or a squatting, or should I say crouching platform. Third, since the cheapest model is plastic, it should last years in a moist spot like a full bathroom. Finally, they did a great job of marketing a line of products for an awkward topic.
On the down side, those who will do their research will find that the squatty potty people admitted it costs them $4.50 if I remember right to make the plastic version. So as far as the cost, they aren't winners over the Welles step people in that respect.
As far as using a step stool, I tried a couple we have around here before deciding to go for the squatty potty. The biggest problem with them for me was that I couldn't put my feet as far apart as I wanted to. This is something the squatty potty solves beautifully for me. My conclusion is that I find the squatty potty to have been well worth the about $20, and as various reviews state, I love my squatty potty. No, I have no interest in the company.
There are as well several options for squatting over the toilet on platforms. Since I didn't want to be perched over a foot off the floor, I didn't personally go for them. I will conclude advising those who want a bathroom aid like the Welles step or the squatty potty, or something else to do your own research before you settle on something, or nothing at all as the case may be.