Depends on your travel habits.
If most of those stays are at properties like HGI and ES's where they don't upgrade typically anyway and you have no use for the diamond force, then probably not.
If the rumors are true that all DT's and Hiltons will have to have exec lounges by 7/31 or 12/31 or you travel to properties with exec lounges a lot then it may be (especially in Europe and Asia). With gold being so easy to obtain, many properties are enforcing the policy of only giving golds access if they on the exec level and there are a lot more golds now to compete against for upgrades, and properties should be upgrading diamonds over golds.
I booked a stay at the Trafalgar in London for this summer a few days ago and being diamond saved me a lot more than $400. The hotel was $409GPB (approx $650 US) and only premium rooms were available. Thanks to the diamond desk they were able to get me that room for an AXON rate. Doesn't work at a lot of properties, but at the hotels it does, it is a great benefit.
You also need to factor in the value of the 60,000 hilton points you will earn from the transaction into the equation as well. Those 60,000 points can be worth more than $400 on some reward stays.