The dividing line between which to use is based on what works in the highest stress environment, from what I read. A very high load requires all the surface it can get on the bottom of the bearing. VERY few of us reach the point where this becomes a big issue. I figure I may be on the threshold at 900 hp, maybe not. The only issues I have ever had with bearings was due to lost oil pressure, and I run 60 PSI max and have trapped as high as 7800 with the 528.
So for my $$, 3/4 if I can get them, 1/2 or full if it is not a real high load environment. The problems would show for most motors if there is not enough of a grove to feed the bearing, so the extra doesn't hurt a thing provided the loads are supported by the surface area that is left.

The bearings I just put in my SB are full groove but
the groove is narrow... this is just a low compression
engine so it doesnt need max surface area for load
and MOST failures I've seen over the years were rod
bearings... very seldom seen mains fail(without some
sort of oil issues)

I think that something is being overlooked here and that is crank rod pin diameter. BBM's have a fairly large 2.375" rod journal which means the bearing speed is higher than that of a BBC's 2.200" journal. Larger journals requires more oil to keep them happy, perhaps why a narrow, full groove main tends to keep a big mopar alive longer. Were you to go to the BBC rod journal, I would suggest going to the 3/4 groove main. As long as the rod is getting oil pressure when it needs it most; ie, under the power stroke, it should be just fine. Ok, just did a little studying of two actual crankshafts, one is a Bryant for an R5 engine and the other is a Scat 4" for a 360. Both oil the same and with a 3/4 groove bearing, the rod won't have oil flowing to the bearing during compression and exhaust cycles. It starts to get oil just about at TDC as close as I can tell. Not sure what this is telling me, sure some engineers could explain it, but I know that the most pressure in the cylinder happens shortly after TDC (unless there's preignition detonation) so with the proper bearing clearances and rod side clearance the oil film is sufficient. Another thing I find interesting is that the oil feed hole to the rod starts feeding the rod bearing after it has cleared the top of the bearing and is at about 3:00 or so. So what this tells me is that having proper bearing clearance and rod side clearance along with proper oil viscosity is very important. Also that rod pin diameter could play a part in whether or not you want to run full groove mains or not.
Found this article interesting: (focused mostly on the cross-drilling portion, though the rest of the article is a pretty good read)

http://www.chevyhiperformance.com/tech/e...ts/viewall.html