RE: Mossberg 12ga Sluggun
I use both, so I will give you my personal opinions on each.
Most inlines will be easier and cheaper to get accurate out to 100 yards and beyond. They are just a PIA to shoot and clean. Also follow up shots are a bit difficult because of the loading process. Most of the inlines should easily shoot 1-1.5 inch groups at 100 yards if you play a little with the charge. Sort of like handloading withought the cases. The down side is the ballistics are simular to a hopped up handgun since most shoot handgun bullets at higher velocities. You could also use the same gun in both gun and ML season in most states.
A slug gun is much cleaner and user friendly in both loading and maintenance. Some can be very accurate when you find the correct load. It might cost you quite a bit to find it though. Premium ammo for a slug gun is VERY expensive, if you are talking sabots. And in my opinion at the ranges you are talking about the shotgun wins hands down in knockdown power and down range energy. Also follow up shots if needed are much quicker and more feasable. However within 100 yards if you put the slug where it needs to be I don't see you needing a follow up shot. Of course the same thing goes for a ML. The down side is you cannot use it in ML season. And where I live you can't use a bow in ML season either. It is for ML's only. I think it is the only season you can't use a bow.
If you took 100 yards out of the equation I think you could even get away with a good smooth bore and rifled slugs. Most shoot prety well out to 70 yards or so. Some even better.
My honest opinion, if you wanted to hunt ML season, I would get a really nice inline and use it in both seasons. That is what I did and then got a nice slug gun later just for shotgun season.
Paul