What's wrong with buying from both? It's not like whichever format loses will become unusable. I'd say if you have the ability to buy either, then don't limit yourself. If something you like is available on both formats, then you can buy whichever format you're partial to. That's what I do. In my own experience, I MUCH MUCH prefer HDDVD... having had players for both formats for over a year now, and no prior biases beforehand, my collection has ended up with 4 HDDVD's for every Blu Ray (I think I'm at like 50 to 13 or something like that). but there's very strong opinions backing both sides. bottom line for me is, if you can play both formats... don't worry about who's going to win. What does it really matter? If HD ends up winning... you can still watch the Blu-Rays, and vice versa. You're in an enviable position compared to those who can only play one format.
If you plan to sit out and determine which one is "better", good luck with that. They're both fantastic and people love one side or the other for various reasons but there is absolutely ZERO conclusive proof that one is better than the other... only opinions. Each format has pros and cons but nothing that tips the scales IMO. Whichever one wins, isn't going to win because it's better... but only because of studio allegiances and marketing efforts. Meanwhile, there's lots of movies out there that you could otherwise be enjoying NOW, if you weren't waiting for the war to be decided. Just my .02 on that.