If you are going to consider a credit card, rather than a direct Skyclub membership, you need to look at the other card features to judge which is the best value for your circumstances. There are several versions of the Platinum Card (standard, Charles Schwab, Morgan Stanley, Business). They are all the same in terms of lounge access, which includes far more than Skyclub, but other features vary. It's also possible to apply for another of these cards each year to earn successive new account bonuses.

There is also the Delta Reserve card which is somewhat less expensive than the Platinum Cards. (Note that Skymiles Platinum does not include Skyclub access, and is not a "Platinum Card" for purposes of this discussion.)