From my experience it would depend on lots of factors such as...
Consider the performance on the underlying storage - more LUNs could potentially mean that you could balance out the IO demands on the SAN more evenly. However, there are overheads to consider with high numbers of LUNs, hard limits and depending on the array, the increase in overheads could rule out any performance benefits.
For your 3TB disks you will need to use RDMs to get around the 2TB vmfs file size limit.
Will you be using SAN snapshotting/replication? If so go for RDMs.
Will you be using physical to virtual clusters? If so you will need RDMs.
My personal choice is that I always prefer to use vmdks if I can rather than RDMs as I prefer the flexibility. I always try and keep the datastores as small as possible, just making them bigger if there's a real need, preferring multiple datastores to one big one. In many of the environments I work on, most vmdks are less the 100GB so I go with a rule of thumb of making my datastores 700GB in size. If I need a bigger one I just create it bigger.
Not sure if this helps as the scope of your question seems quite wide...