I'd recommend ATP. They're kind of a diploma mill, and I wouldn't necessarily recommend them for primary training, but I will say they know the Seminole and they know multi-engine procedures.
Nothing will be easier than doing it in a TwinStar (except, perhaps, an AirCam), and there's an argument to be made that if you're headed for a jet of some sort, the more modern Diamond is closer to that.
But there are some things you simply won't learn in the DA42. There's a big difference between having two levers in front of you and having six of them. You only get one chance to grab the correct one. The TwinStar feather itself, which makes the checkride easier... but if are going to be moving up to a more traditional twin (Comanche, Twin Beech, King Air, Conquest, etc) after getting your multi-engine rating, you'll be better prepared if you take the multi-engine training in something like the Seminole.
You can do the multi- rating in a long weekend; one of the reasons I selected ATP for my own multi-engine rating is that they have variety of locations and a large fleet of Seminoles. When one breaks down, they just move you into one of the other examples sitting on the ramp. So mechanical issues, should they occur, are less likely to impact your training.
If you'd like to read the writeup of my 2004 multi rating at ATP, it's available here:
http://www.rapp.org/archives/2004/07/multiengine_rating/
Good luck. We're all counting on you.
--Ron