JWP,
Old Buddy, . . . if you are trying to decide between a .30-30 Marlin and a .30-.40 Krag, then you need to take a time out for a few weeks on this decision. You are comparing apples and oranges, they are totally different rounds with totally different strengths and weaknesses. The rifles are also totally different.
You can never go wrong with a .30-30 Marlin, if you keep your shots under 200 yards. There is a reason why the .30-30 Marlin 336 and Winchester 94s are the most popular rifles in America. They are like the F-150 and the Chevy C-10. They just plain work, and they hold their values very very well.
The .30-40 round is much more poweful, and much more expensive in factory ammo. The only reason to get the Krag is if you just love the Krag bolt action and can't live without it.
So, you have to ask yourself why you are buying the rifle to begin with. Is it to have something cheap, and effective and popular to shoot? Go with the Marlin. If it is because you love the history and action of the Krag, and are willing to put up with special factors regarding the round, then buy the Krag.
Although the .30-40 Krag is a good round, the common old .30-06 is better and is much cheaper in factory ammo , and you can get a good .30-06 bolt rifle for a fair price anywhere.
If you want to split the difference between the Krag and the .30-06, then go with your gut and get the .303 British Enfield. A great gun. But, good factory ammo can be expensive.
Mannyrock