Asking what kind of bullet to buy is like asking what kind of shoe to wear. It depends.
For target practice with either weapon, I'd recommend buying inexpensive brands such as the low end PMC, Sellior & Belloit Winchester White Box, etc. For serious applications - say competition - you'd already be reloading. For hunting, I'd go with "match" ammo.
For the elusive Zombie or SHTF scenario, you'll want something like the Black Hills you have, or another reputable brand such as Federal or Prvi Partizan "Green Tip."
Especially for your Sig 556, I'd watch buying factory reloads or Wolf (or any steel cased ammo). Many of the AR-15 variants have a hard time with Wolf because it uses steel cases. I'm just starting to reload, but I've been told that the 5.56/.223 is one of the most difficult cartridges to reload and the 5.56 AR-15 variants are extremely picky about ammo. Many of the factory reloads, for whatever reason, don't meet the standard.
The steel cased ammo works well in some weapons and will barely function in others. If you try it, you'll find out real quick where your gun is at. For example, I have a 9mm XD that has about 1 malfunction per 50 rounds of Wolf. I've shot about 2K rounds through it. My buddy has the exact same gun, but has shot about 20K rounds through his. His will eat Wolf. My Ruger P89/90 also eat Wolf with no problem.
For ammo purchases, I'd try Ammoman.com, Aimsurplus.com, wideners.com, natchezss.com, or Surplusammo.com. I've done business will all of these companies and found that they reputable. YMMV.
