The .450 caliber has a few larger bullets available, 400 and 405 gr. but, when loaded with the same weight bullet weight, the velocities produced are very similar between the two.
444 Marlin:
http://www.reloadersnest.com/query_bw.asp?CaliberID=90&BulletWeight=300
450 Marlin:
http://www.reloadersnest.com/query_bw.asp?CaliberID=196&BulletWeight=300