As mainaman says they are different shapes and leave edges with a slightly different feel. I have both and I'm glad that I do. IME sometimes one is more effective than the other depending on the blade and the circumstances. They aren't that expensive so more options are worth the try.
If you are outside of the USA and it is more convenient shipping to get the TI 4 sided strop than go for it but if you are in the USA go for the Tony Miller 4 sided paddle
here. Great workmanship and he can furnish the diamond pastes too.