My thoughts on the Bangai-O series. I played the Dreamcast version first. Loved it so much I decided to buy the n64 one. And the same for Spirits.
DC:
If you're interested in the original Bangai-O, it's much easier/cheaper to get the Dreamcast version. The versions are slightly different, but overall I'd recommend the DC version for price and ease of play. You could probably get a DC and the DC version of Bangai-O for the price of the n64 cart alone.
The game itself is amazing. I was a bit put off by screen shots and the size of the characters in this game, but once you find the quirk that makes this an excellent game, you'll see why they are so small. You play though levels one at a time and there is a crazy story line loosely strung throughout.
N64:
The n64 version is still a great game, but it's completely in Japanese. So learning what exactly to do is a bit difficult. The n64 does have interesting controls though. You can use the dpad to steer and the analog stick to shoot. It's weird at first, but once you get used to it, it works very well. After awhile of playing with these controls, I liked them a lot more than what was available on the DC. There are different controls options for both versions, so you can choose what you prefer. But it's basically a two stick shooter that has never been released on a system with a two stick controller.
DS:
This game seems more like an add-on pack to the original Bangai-O than a full sequel. There's not really any story, you just attack the 100+ levels in any order you want. There are puzzle stages which require specific actions to complete. Then there are other levels that seem to be the best the developer came up with while messing with the included level editor. It's a really good game, but I wanted more 'game-y' stuff like level progression, story, and difficulty progression.