I blogged several months ago about devices and gadgets that I put my hands on to make push-ups harder. I also talked about this in a forum and I stated that I believed that rings could make the push-up harder than the T's. There was some friendly doubts about my statement and I figured that after working with both some more, it's a good time to revisit the topic.
I'll come right out and say it: superiority of one over another is murky. One problem is how each piece of equipment is set up. I built both my rings and my T's. The level of difficulty of the rings depends on how far apart you space them. The farther they are away, the harder they get. With T's, the length of the stilt and the width of the foot makes a huge difference. The higher the stilt, the harder it is. the wider the foot, the easier the push-up becomes. So, it's hard to deduce which is harder just by design.
What I think makes one easier than the other is what you're trying to work on. T's require a tremendous amount of grip strength in order to stabilize the movement. So, these are definitely superior for building up your grip, forearms, biceps and triceps. I think that these are a bit more difficult when it comes to core stabilization as well than the rings are.
Where Rings aren't as hard on the arms, they're a bear for the shoulders and the chest. I notice that they're noticeably harder on the chest muscles than the T's are, especially if you do them Atlas style. What's interesting is that if you count reps as an indicator of which one is harder, then rings are harder for me. I can do more push-ups on the T's than I can on the rings.
So, what are you looking for in your workout? Where are your weak points? That will determine which is harder. If you want to favor your arms over your chest, go with the T's. If you want to challenge your chest and shoulders, do the rings.