The 16V is a little quicker than the 2.6, but the primary difference is gearing. From about 80 MPH it's close to a wash (as is top speed), but better acceleration from lower speeds is more useful. With equal gearing low speed acceleration would probably be a wash. Mercedes intentionally installed a small 103 engine and tall gears in the 201 so it would be slower than the 16V and 300E. The ultimate Merc would be a 190E 3.0 with the 16V direct drive five speed and a 3.07 axle. I seriously thought about doing this a few years ago, but decided it wasn't worth the cost, but boy, what a sleeper because it would look absolutely stock/original to the unsuspecting eye.
Keep in mind that the 16V is a more maintenance intensive engine with shim under bucket valve adjustment, which requires removing the cams to change shims, and MB recommends a lash check every 15K miles. The hydraulic lifters on the six are maintenance free. It's also my understanding the the 16V rear leveling system is problamatic and very expensive to repair.
If you can find a 2.6 with a manual transmission it's a pretty sweet and smooth package, especially if you do a lot of touring. As previously stated, the 16V is edgier and would be a quicker canyon carver. My 2.6 five-speed is a pretty decent back road athlete, and I can also drive 1000 miles in a day in perfect comfort and not feel beat-up after 16+ hours of driving with nothing but fuel stops and a quick lunch break.
After my '87 2.6 five-speed order was rejected by MB when they decided not to offer a five-speed after initially announcing it would be available, my dealer offered me the 16V on their showroom floor at a substantial discount. Had it been red (16Vs were never available in Signal Red in the US) I might have done it, but decided to wait until 1988. Fortunately, the 2.6 with a five-speed was offered as a special order option, so I ordered it, took delivery, and never looked back.
At Starfest '92 the 16Vs had their way with me on the drag strip due to their shorter gearing and LSD, which allowed them a hole shot. In the autocross two would have beaten me, one by less than two-tenths on a one minute course, and I think the winning car was modified as it had a two second advantage. On the two minute road racing course I would have just barely missed fifth place. As it was, after expecting to be classified with the 16Vs they broke the six attending 2.6s into a separate class, and being as how I was the only guy with a five-speed and racing experience, it was pretty much a rout.