Depends on what you want to use your miles for. If your looking for award tickets, you might want to bank with Delta IF the airlines and ticket types you buy will give you 100% mileage (handy chart link: http://skymilesoffers.delta.com/airline_partners.php). FB only gives you 50-75% mileage on most tickets unless full fare.

Delta's web tool is useless for most SkyTeam partners (and pretty useless in general), but I've found for KL and AF at least award availability to be outstanding. Also, if its a DL codeshare, you earn on the DL earning rules vs. partners, which may help a lot depending what routes you fly.

One bit of advice: If you bank with Delta, don't bother ever try redeeming a DL award; its pretty much impossible at LOW levels. :-/.