In general, you should pick one, because that way your miles will be consolidated and you can earn elite status more easily.

However, on this point:
...depending on the offers available to you, it probably makes sense to get both. And to get both, you'll need to have memberships in both programs.

Also, occasionally, either Alaska or Delta will offer promos to their members only. One I recall, in particular, was 25k Alaska miles after a single round trip out of one of the SF Bay Area airports. (Maybe 2 years ago?)... You could turn a <$100 trip to Sacramento into a trip anywhere in the country...

Perhaps a better question would be: is there any reason NOT to have both accounts? Probably not. (Occasionally there will be new member enrollment bonuses but that's rare.)

As an aside, if you'll be looking for elite status, you'll probably want to familiarize yourself with the MQD (Medallion Qualifying Dollars) requirements for Delta elite status starting in 2014. There's a lengthy thread in that forum.

EDIT: Here is the promo I remembered... almost how I described it... http://www.flyertalk.com/forum/alask...promotion.html