Bird was awesome. But in terms of both achievements and longevity, Duncan has had the better career (and I don't think it's that close).
Similar per-36 numbers, but:
les: Duncan 5, Bird 3
MVPs: Duncan 2, Bird 3
PER: Duncan 24.5, Bird 23.5
Defensive Rating: Duncan 95.6, Bird 101.4
Win Shares: Duncan 201.2, Bird 145.8
and perhaps most importantly:
Games: Duncan 1331, Bird 897
You are basically picking between two players of very similar production, but Duncan edges Bird in most categories and has done it for 48% more games than Bird did. So if picking between the two, I don't think it's really even close.