Both Intel and AMD are fine... it's like asking whether a Ford or GM is the better car. I've used both with great results over the years, and would be perfectly happy with either set-up right now.
Performance-wise, there is little to no appreciable difference between the T-bird and P3... some benchmarks favor the T-bird, some the P3. The duron vs. celeron, though, is another story entirely... the Duron absolutely smokes the Celeron. And it's not even close. And both AMD processors are cheaper than their Intel counterparts.
In my opinion, the Duron paired with the MSI K7T Pro and Mushkin sdram is the absolute, far and away best value per dollar solution out there right now. I have the K7T board, and it is absolutely indestructible. Most stable board i have ever used in my life, and i have prior experiences using Asus, Abit, Tyan, Gigabyte, you name it. I highly recommend it.