No such thing as a brush gun, all guns will deflect on brush or branches. this is myth that I wish someone had not started, the same as knock-down power, another myth.
Your 6 mil would be my choice as for reasons stated above.

by" brush gun " an ounce of lead will have less deflection than a smaller cal. bullet like the 6mm personally I'd go with the slug or maybe 30/30

johnl yes the smaller bullet will have more deflection but it is also sneaks though a lot of brush that a 30 cal. would hit.

i would use the rifle over the slug in the thick stuff. but i would try for a high shoulder shot, neck shot, or a spinal shot, all three will drop them in there tracks

I love my slug gun. Smoothbore, and I shoot clover leaf groups at 50. If the range is short, thats my preference.

I'd go with the slugs. more weight thru the brush

If i was you i would use a rifle. It also depends on the caliber you are shooting. I shoot 165 grain 30-06 and i have no problem with brush. also my dad shoots a 30-30 which is a really nice brush gun. But i personaly think that 100 gr. is to little and would suggest to use a higher such as 130 or 140 if u still wanna shoot low grain. if you dont care i would suggest 150 or 165 grain.

The 6mm or slug gun are both adequate for the job. In fact the 6mm vs. a standard sabot slug have very similar energy up to 100 yds. My vote is for the shotgun but with a premium bullet like Federal's Barnes Expander, Winchester Partition Gold, or Remington Ultra Core Lokt. FYI the Partition Gold kicks like a mule. I just switched to the Barnes Expander. Good luck.

If you shoot the slug gun well out to your max ranges by all means use it, otherwise stay with your rifle.

I live in Central Indiana. In Indiana it is illeagal to use rifles. So i have to use my slug gun. Where i hunt it is pretty thick in most places. I use my remington 870 express magnum. I have never used a rifle to hunt deer so i can't really judge them. But I always have had great luck with my slug gun in the thick stuff. I haven't had to track one yet. Drops them right in there tracks.

ruger,
I love my 2 sluggers,have taken many many deer with both. Great guns in woods if 00 is legal in your state.

I started deer hunting with a rifle. It spoils you after your forced to switch to a slug gun! By comparison all things considered a shotgun IMHO is a big panty waste. Every year I wish I had a good rifle in hand and in many cases could of taken shots on nice deer at 150-200 yrds. I then look down and there sits the old slugger.