Phaneuf is a #1 defenseman at the age of 23 with the potential to be a team maker/franchise player. He could even rival Scott Stevens with some maturity. Burns is a #2 defenseman IMO, he might do ok as a #1, but he's no Chara, Bouwmeester, Pronger, etc. If he makes improvements even close to last years, he'll be right there, Phaneuf already is and that's the difference.
I think it's all a matter of time. Phaneuf hit his growth peak the year after his draft and has improved immensely every season, meanwhile Burns started hitting his growth streak about two years ago, and IMO, Burns' improvement on a year to year basis is much more impressive than Phaneuf's. Still, Phaneuf is better now, but I think Burns will be better in the long run.