Roamio, as compared to a Series 3

Could someone tell me what I would find in a Roamio, compared to a Series 3?



I currently have several Series 3's. I call them "rescues", they were broken liftime units on ebay. I'm using them for OTA.



I recently got an email about amazon cancelling their video download service, and because I had used that service on a S3, they sent me the email, and it includes an offer for $100 off a roamio.



I'm thinking that is almost just enough to get me to try one.



I am OTA right now. There is the possibility of cable in the future, but for now it's OTA, which means I can only use the 4 tuner model, correct?



Will the Roamio transfer shows between itself and the series 3 units, or is it's multiroom capability limited to streaming to minis? It would be great if it could do both, but I have a fear that may be too much to ask.



Thanks