For .303:
I've got both Greek surplus and South Arfican surplus. I've shot up probably about 400 rounds of the Greek, and it's been flawless (it's reloadable, too). I've only used a few rounds of the SA, but it's been fine so far as well. Of the two, the Greek looks better - brighter and less tarnished. It also came in bandoliers and stripper clips. Unfortunately, I don't think there's much of it available. The SA is really cheap - I got mine from AIM Surplus (
www.aimsurplus.com) for $7/box of 50.
The one kind of .303 I would avoid is AmmunitionStore's .303 AP. I bought some and sectioned one of the bullets - it's actually pulled .30-06 AP bullets reloaded into WWII-era .303 cases. It should work decently in a good rifle, but I wouldn't chance it. Unfortunately, genuine .303 AP runs about $1/round when you can find it.
My .308 experience is limited to Portugese, which has worked fine in my Vepr .308. It's Berdan primed though, unfortunately.