Here's my $0.02:

The Hornady 220 gr RN is closer in dimensions to the original bullet, although it's not a FMJ.  The Sierra is a good bullet, but has a slightly different shape than the original.  Unfortunately, the only bullet that is almost exactly right is the Woodleigh, which is insanely expensive!

The bottom line is, as Whig and Culpeper have said, try them out and see which one your Krag likes better.  I have shot hundreds of both, and my Krag prefers the Hornady.  Others like the Sierra better.  Krags are practically human in their love of food (ammo). 

Here's my load:  Hornady 220 gr RN over 40.0 grains of IMR-4350 and a CCI #34 Mil-Spec primer.  I'm looking to possibly switch to using H4895, as the 4350 burns very slow, and I tend to have unburnt powder exiting the barrel.