I don't think there's a practical answer from the software side here as it's all about where you want to focus your management efforts.
RDM is useful because you configure once at the host level and get all the benefits and don't have to deal with the network abstractions that might kill off some features you rely on.
iSCSI is "simpler" to set up initially
Performance wise I think at this stage in HyperV and VMware you should get pretty good performance on both sides, but some things to consider (in no particular order):
- Performance is about equal
- Not sure if this is true anymore, but I think live migration becomes a concern with RDM if the other hosts don't also have access configured to the storage but worth investigating since you don't want your repository bound to a single host
- iscsi can be detached and reattached to new machines pretty fast, RDM will take some configuration time
- you need to check the network at two layers with iscsi; the virtual networks and the physical (i.e., MPIO is useless if you set it up in guest but only have a single NIC over which the traffic might leave the HyperV environment)
- A lot easier to reboot a virtual machine than a host for patching issues to correct iscsi bugs (MS has tons of them over the years)
Ultimately, "pick your poison"
You'll have some management considerations with both approaches, and it's just whichever you feel more comfortable with.