Yes, don't compare a slug gun to the 45-70. The 45-70 is a significantly better game killer. Slugs are not going to provide deep penetration like a 45-70 bullet. Especially large thick hide game like a griz.
Many here may push slugs as big game killers because of the stats on ballistics charts. In real stopping power, a slug is second fiddle to most big bore and med bore bullets of high quality construction. Although the charts look good on slugs, you will find that the manufactures don't push them as medicine for large game such as griz. Most companies push slugs as deer medicine and perhaps for small black bear. I would prefer a 30-06 with a 200 or 220 grain load for griz medicine over any slug, period.
Another source for second opinion will be a pro outfitter in griz bear country. I think you will find that these guys don't push slugs as griz bear stoppers and with good reason. Most pro's will look to 338 cal and up in high speed rifle rounds or big bore rounds like a 45-70, 458win, 416rig, ect.
The proven "package" of the guide gun is really geared towards your specific needs in mind. Marlin calls it a guide gun for good reason. They wanted this package to appeal to Pro Guides as their weapon to back-up their clients. When the client fails to hit the game (or hit well) and its coming in for its own kill on the client, the guides gun is now "up to bat". The small light weight, fast steering, quick cycling guide gun with a 458cal bullet of 400+ grains in a hard cast load will stop nearly anything ugly that mother nature can unleash.
I am not a pro guide nor a Marlin sales rep. But I do own a Guide gun and understand its characteristics and capabilities and place it much higher on the list than any slug gun.