I'd vote for a new SST - there will never be another Concorde. It will always be a marvel that so much was achieved with the technology available.

I suspect the biggest barrier to a new SST is the sonic boom. Everything else modern technology could do better than Concorde - the dev costs would probably be lower because all of the hard science has been done. Is there any possibility of a shock wave cushion (patent pending) to surpress the boom?