If youre a DIYer, go with the refrigerant weight. Pressures are not reliable for filling it up completely, and you could miscalculate oil amounts too. If youre filling it from an external tank (aka, an autozone A/C "booster," talk to them about how many cans are needed for your specific refrigerant weight.). Best bet, let the pro's handle A/C charge, cause shops have special machines for it.