Uh oh, do I hear a very large can of worms being opened?
FWIW: I'm a Winchester man myself. I've got nothing against Marlins, they're good rifles. But, for me the Winchester handles better plus they look like the traditional cowboy rifle.
Marlins are easier to mount a scope on. JMHO, putting a scope on a lever gun amounts to blasphemy. I wouldn't dream of it. However changing to a better open sight or a peep sight is a good idea.
As to the choice of caliber, it depends. If your shots will be 100 yards or less, get the .44 magnum (or my favorite, .45 Colt). The advantages are less recoil, wider variety of ammo, higher magazine capacity and it's much easier to reload for.
If your average shot will be 100 yards or more, The .30-30 is the hands down winner due to its better long range ballistics.
Ok, I'm puttin' on my asbestos underwear, so all you Marlin lovers flame away!