The final form your film goes back onto is up to you. Digi-Beta is a good tape to master to, because it's very, very good at retaining information, plus it's the TV industry standard format (UK).
However, there is no reason why you couldn't master to HD, providing you did your online on Avid Nitris. I think the decision you make will depend on your final end market. If the film is going directly to DVD or TV, then layback to Digi should be fine, if you are looking for a cinema release a layback to HD and a layback to Digi-Beta is a good idea. You'll need your Digi copy for running off tapes/DVD's to send out, simply because you HD deck is a fortune to rent. We were paying $1000 a day to rent ours.