With the ML you'd have a a much better choice of pointed bullets with better ballistics than what you'd have with the 12ga slug. Anything from a 175gr to around 340gr or so, and they all are longer slugs with better BC than what you can get with a shotgun, with a couple of exceptions:
You can get a 12ga SST slug with the 300gr bullet at 2000 fps, and the new Winchester Elite 12ga 2 3/4" and 3" slugs are 2000fps and 2100 fps, respectively. I'd venture that's probably the flattest shooting slug you can get right now. Remington has a pointed slug, but it's a .58 cal 385gr at a slower speed, so I don't think that it's going to have a flatter trajectory than the others.
All of the other shotgun slugs are a heavy, blunt faced slug with poor long range potentional, compared to the above. Don't get me wrong, a heavy blunt piece of lead is still a killer at 200 yds or more, but with the arching trajectory, 1st round hit potential is going to be very limited.
There are lots of people on here that have taken deer past 250yds with their ML, and one guy that I know of (Andy) has killed deer at over 300 yds with his. Lots of us shoot targets at 200 yds or more with our ML'ers. We practice those longer shots so that we know we are capable of taking them under field conditions.
I don't know anybody personally that's hit/killed a deer past 125 yds with a shotgun slug, and I also don't know anyone who practices shots past that range, which is the most important thing.
The bottom line is that both will kill deer at extended range, given an accurate load and an accurate shooter. IMO, though, you're more likely to do with a good bullet in a ML than with any shotgun slug, because you're going to have a flatter shooting bullet with better accuracy than what you generally get with a slug gun and slugs.
There's not going to be any definite range limit, because the limiting factor is going to be the shooter, not the equipment. I'd say that my max right now (because I just haven't taken much time to practice this year, with work and all) is going to be around 200-225 yds or so with my T/C Omega. Last year, I practiced much more, and I would have gone 300yds without hesitation. If I was using an USH with one of the new pointed loads, and the accuracy was the same as my ML with a similar trajectory, I'd have the same limits for myself.
