If you perform well with him then keep rolling with him. In Ranked, I'm not sure how many people actually run back-up catchers. Especially with the Extra Inning rule now in effect. It might be more beneficial to have an extra pinch-runner as opposed to a second catcher. So if you do well with Contreras just keep running him out there and levelling him up. That's one of the great things about Parallel is that even those Gold cards can become marginally better and put them on par with cards that are technically "better."
I seriously struggled to put Piazza in over my Level 4 Will Smith. In 36 games online he has 15 HR in 50 AB, hitting .500/slugging 1.500 with an OPS of 2.010. Then when he's playing up on top of it he's actually a gross little monster.
As the year goes and you earn "better" cards or pull them your decision will be easier. This early, though, there's really nothing wrong with a Gold. You could always grind out for the 42 Series Perez too and go from there.