I'm doing a lot of travel for work to various locations, and for most of the flights I have a choice between United and Delta. (Estimating around 60k miles per year.) Is it better to concentrate all this mileage earning on one airline (likely United) and go for higher status tiers, or should I try to go for the basic level of status on United AND Delta?

There are three things I care about when it comes to status, in decreasing order of priority:
- Earning miles that I can use for cheap economy award travel (to international locations, mostly).
- Getting free access to Economy Plus seats.
- Being able to book economy award travel last minute, make changes, and cancel/redeposit without huge fees.

I don't care about free checked bags, priority boarding, priority security, etc. and I also don't care that much about flying in premium cabins, particularly when not traveling for business.