The big difference is how much of the 61.129(a) requirements have you already satisified in the 210TT you currently have? Chances are that you'll only need 10 hrs of complex instruction (where you'll learn to do the commercial maneuvers, and a day/night 100nm cross-country) and a 300nm solo cross-country.
If you take the multi-engine route as your initial commercial rating, you will have to satisfy all of 61.129(b). (At a minimum, that's 10 hrs of instruction and 10 hrs of "solo") If you do it as an add-on then you have to satisfy the requirements of the PTS. Most pilots can get an add-on ratings for 10-12 hrs, although you can certainly find places that advertise a package deal for less flight time.
Many acadamies give you a commercial multi-engine initial rating because they want to maximize ME time. If that's your goal, then this is a good reason to get a ME rating as your initial commercial.
Either way, I would recommend you don't get a private ME add-on if you're going to get a commercial rating. You can get a ASEL commercial rating, and add-on a ME rating to it at the commercial level as your first ME rating if you want to minimize expense. You can also get a ME initial commercial and add-on a ASEL (if you need it).
Unless you have a good reason to justify the expense of taking another checkride at the private level just to add a MEL to your private certificate, or checkrides are your hobby, just skip it and make whatever commercial ME route you pick your first ME rating. (I got a ME add-on to my private because I got some bad advice from my CFI. It was $ I didn't need to give the DE because I ended up taking a commercial ME add-on. I could have done it all with that one ride.)
When I was working on my CPL, I did my 10 hrs of complex training in a twin. You can even use it for the complex proficiency part of the check ride if the examiner agrees to be the PIC. My CFI told me I couldn't do that (although I later found out from the FAA that I could have), so I spent about 1.5 hrs in an Arrow in the traffic pattern and used it for the test to do one touch & go and one full stop. I did the rest of the test in my own Yankee. The Arrow was $110 an hour and the twin was $170 (years ago) and I really didn't need 10 hrs of instruction in the Arrow since I was going to do the test in my own plane. I had flown Warriors before, so the transition to the Arrow was simple. There were just 2 more levers to worry about and an RPM gauge that didn't work like a Warrior's. But I learned all about props & landing gear in the ~15 hrs I spent learning to fly the twin as a private pilot.
Rob