It was announced (last year?) that HD-DVD was going to be discontinued because the popularity of blu-ray was overwhelming everything else. Besides the better resolution that @whackyrusty already mentioned, blu-ray discs can hold massive amounts of data (up to 50 gigabytes) which is six times the amount found on conventional DVD discs at the current time.
Blu-ray discs are also much more scratch resistant than regular DVD’s and blu-ray disc players will accomodate both blu-ray and regular DVD discs also in all the players that I have read about.
As far as a sound investment right now, I don’t know. Blu-ray players aren’t the end all be all of home entertainment at the present time but their popularity is predicted to go way up in the coming years and the technology certainly is better than current DVD technology.