SSIA, in short. I'm Alaska now but should I enroll in Delta as well?

Flying out of PDX for the next year, possibility moving to New York at 2014 year's end. While I'm here in PDX I mostly fly Alaska but longer flights to see family are on Delta (or others, likely American or SWA). Plus, I may move out the PNW next year, and definitely will in 2-3 years.

So, is there any benefit to being in both programs? I know they're partners and there is a lot of reciprocity, but are there any additional benefits of being in both? Secondly, with this stuff in mind, should I be getting a Delta credit card or Alaska credit card?

Kind of a rookie lurker here so don't kill me too bad.

Thanks.