I just ordered smooth orange t-molding from t-molding.com, and smooth black t-molding from Happs and have installed it on identical pieces of 3/4" MDF.
The t-molding.com stuff fits perfect and installed perfect (into 1/16" slot). The Happs stuff was difficult to install (didn't want to squeeze as easily into the 1/16" slot) and wasn't exactly 3/4", some of the edge of the wood shows (just a little, less than 1/16" for example, and is inconsistent at that).
I would have to say the t-molding.com stuff has worked out better for me too.