I too am a big fan of TOBS Jermyn Street. The ingredients are actually the same for both, see below.
I can't say why but the scent of the aftershave seems to hang around longer on my skin.
Also, to my nose they smell the same.
Just in case, here are the ingredients of the aftershave balm.
I have the cologne, and its a fantastic scent that lasts surprisingly long, for a cologne. Lasts longer than many of my EDT's.
This is a little off topic, but I don't know where else to put it. If you like Jermyn Street, pick up a stick of Old Spice Game Day Deodorant. I was shaving with some Barbasol original foam today (I don't use it very often), and I could swear I smelt ToBS Jermyn Street coming off it. Finally I realized it was coming from my new deodorant! Its actually uncanny how similar they are. I didn't notice the similarity in the store, only when I put it on.