The DM-650 and DM-670 are the most commonly used DVR's in DSA. The DM-670 has larger 8GB storage and up to 999 indexes. The only difference with the DM-901 is that you get the new style screen menu system, rather than older LCD display and it supports wifi indexing (though this won't work with most uni systems that require logon). It a bit too complex to use for some; and the downside of the DM-901 is half the storage space of the DM-670 at 4GB.
I have a DM-450, DM-650 and DM-7 sat on my desk and I use the older DM-450 the most (but you can't get them now). In my opinion, the DM-670 is a better model.