They're going to be well within a few percent of each other. The Philips bulb will win out on filament precision (that is: beam focus). Beam focus ultimately gives the beam more punch and distance (assuming proper aim) and reduces glare for other traffic (again, assuming proper aim). Between the two, perhaps the $18/pr price difference could mean you're really spliting hairs when that $18.00 could be doing something useful like putting a few gallons in the tank or added to your next car payment as a principal payment.
Whenever a single bulb goes out, replace the pair and keep one of them as a spare. I suppose you could keep whichever one as a spare and put it back away again-- the spare is to get you around long enough to replace the complete pair again. I mean, you COULD always keep the remaining one from each last pair and discard the other working ones but there's no point into getting that painstaking about it.