Do you use the OTA tuner in the 612 for some of your recordings? If so, remember that each HR HDDVR from Direct can only record 2 events at any given time. So if you get 2 HRs, the max you could record at one time is 4 events. If you are getting the whole home dvr, 2 HRs might be just right. If you're not sharing video, then you might consider a 3rd one.
My useage when I was with Dish was that I often had 3-4 recordings going at once during prime time. When I switched, I got 2 HRs, one for me and one for my adult son and we signed up for whole home dvr. I found that for me having only the 2 tuners available to me was just not enough, so I got a 3rd one and stacked it on top of the original. So now I have 4 tuners for me, and 2 for my son.