i just got another bladder from outcast for some reason, this means i got 3 working bladders. my original air cell is made of urethane, the other is the one they sent me, a vinyl bladder, i got 2 vinyls now. im wondering if i should take out the urethane replace it with the vinyl or leave the urethane in. which is better?? or does it not matter, and i should just keep the one i got today as a back up? David C