Isn't there gonna be cross platform play with 360 and xbox one players? Please correct me if I'm wrong. If Halo 3 plays better on the Xbox one I'd definitely pick one up. I guess we won't know the full details until it gets started closer to the date of bc.
Yeah the BC version of Halo 3 will play on the same servers as the 360 version, so XBone and 360 players will play together.
Advantages of playing on Xbox One over 360
1) Screenshots/Video Clips/Twitch/Mixer - I know Halo 3 has a built in system for some of these but playing on the Xbone will allow you to share content much easier. And from console streaming will be completely new. These can all be uploaded directly from the console too.
2) Xbox One Party Chat - The Xbox One has a higher quality party chat. So that sounds nicer. It will also work to cross chat with anyone playing other xbox one games. (Note: xbox 360 party chat is still there if you want to chat with someone playing on 360. And there is an option to quick jump in to game chat without dropping from the party)
3) Custom Layouts/Elite Controller Support - If you have an elite controller you can set up your own controller layout. Swap around any buttons you want. You could also change the analogue sticks and triggers too. Make them more responsive.
4) Clan/Club Support - You can join or make a club/clan to find other players to play with. There are several Halo/Classic Halo/Halo 3 clubs already. In the future we will be able to set up Tournaments too. The Classic Halo Club became really toxic towards Halo 3 players though so I started a new one called Haven Arena
. But through these you will be able to schedule sessions and always find someone to party with.
The Xbox One has had several major updates in the last year or so. It's fixed some stupid design choices, added back in missing functionality, and streamlined the new additions. So yeah I do think playing on the Xbone will be an improved experienced over the 360.