There's no difference in resolution. What you're asking is the difference between HD DVD and Blu-Ray, I believe. The difference is in format, not quality. They're both equally, high-definition formats. They're just made from two competing companies.
Blu-Ray just won the format war. Most of the studios are going with them, and Netflix, Wal-Mart and Blockbuster recently announced that they're not supporting HD DVDs any longer. HD DVDs are going to be stopped being made. So, go out and buy a Blu-Ray player and blu-Ray discs without a second thought. Hope this helps!