[Plums]

The differences you are seeing between advertised and .050 durations between manufacturers, are the differences in the ramp profile (the steepness of the rise up the cam lobe). If the .050 and advertised numbers are closer together it will have a steep ramp, further apart will be a shallow ramp.

Generally, I would go for the steepest ramp for a given duration and lobe separation. This will generally give you the "best manners" (idle and drivability).
One thing to note though: When using steep ramp profiles you must ensure the rest of your valvetrain is up to spec. Steep ramps can be particularly tough on valvesprings.

Edit: Looks like blykins beat me to it, except that if the adv/.050 durations are really far apart they will have slower valve lift. In fact cams where the adv/.050 numbers are a long way apart can be real dogs down low and at idle, (not much fun to tune either).

EG. 280/290 adv with 240/250 @ .050 has steeper ramps than a 290/300 adv with 240/250 @ .050