I believe they use the same bullet. The 450 is a little faster, thus shoots a little flatter. I doubt there is much difference at realistic ranges. Neither is meant to be a long range round. The cost of slugs is much higher but you can buy a lot of slugs with the money saved not buying a new gun.
With a 22" barrel the 20ga Accutip is 1850-1900fps, the specs on Remingtons site has the 450BM at 2100fps but that's out of a 24" barrel, out of a Ruger Ranch with a 16" barrel probably gonna be less, don't think it would make much sense for me to buy a 450BM seeing as how in the real world they would probably have same result on a Deer.