Depending on barrel length, powder charge, etc. One should be able to get 1850fps out of a 500 gr bullet in the 50 Alaskan. Personally I would go with the B&M version as there are more buller options and cutting edge bullets in .500 are cheaper than .510.
Of course you could be like me and have 510 Kodiak Express on a Marlin pushing a 500 gr bullet to 2150+! If you wanted to have some fun that is !
When in doubt, get all the above! The way I look at it, when you get a 45-70, you'll fall in love. Then you'll realize that more is better and an Alaskan will be your next move. Then those Turnbulls are oh so pretty, and one more lever could never hurt!
Ask me how I know, I'm on my third custom Marlin and looking to do more. Thinking about a takedown on an 1894 with 3 barrel sets: one in 10mm, one in 9x25 Dillon, and one more in 7.5 FK. Might even have to look at a .25 option as well, a man can dream!