My (admittedly very crude) schema to explain it to others is this:
-Silver is good for exit row seats and Comfort+ maybe 30-50% of the time and the chance First upgrade (maybe once per year)
-Gold is good for fairly reliable Comfort+ upgrades and off-chance First upgrades (say, on a fairly an empty flight).
-Platinum is good for instant C+ upgrades, somewhat decent First upgrades (especially on not so competitive routes) and RUCs.
-Diamond is good for instant C+ upgrades, Zone 2 boarding, and better First upgrades and RUC/GUCs.

Yes, the upgrades are very route and timing dependent, so don't throw tomatoes at me for this crude characterization!

If you have a choice--and especially if you're booking using OPM and Delta flights don't cause you to have to take a connection (over direct flight with another carrier), I'd go for Gold for more reliable C+ upgrades and SkyPriority access (the latter being most relevant if you're at a hub and might check bags; at most outstations, SkyPriority line includes Silver Medallions).