I like the style of the Tharpe. Also, the Mercer is not very functional, from what I have read. The compartments are quite shallow, especially the side one which can't hold much at all. When you put the bag down, the magnets holding the side pockets together are quite weak and open up, spilling anything in them. Thus, you are only able to use the middle compartment. Try doing a search on the Mercer. I had considered it at one point but after chatting with many tpfers decided to go with something else.
They're both wish list items for me but as time goes by the Tharpe beats out the Mercer, for functionality and I think maybe it looks a little smarter too, I'm going off the Mercer look as time goes by (glad I didn't get one!).
I like the shape of the Tharpe better as I tend to prefer bags with more height to satchel shapes. However, I think the Mercer is a bit more comfortable to wear over your shoulder (based on having owned a Mercer and having tried on a Tharpe). I believe that with the fall bags, the Mercer leather is much firmer and will not "spill" open when you put it down.