Halo is a wonderful series. Battlefield is also a wonderful series. However, if one must decide, I would have to go with Halo. Not only is there substantial lore and extensive plot involved with the Halo series, the Halo games themselves offer a little something for everyone. You can invest in a number of different difficultly levels throughout and extensive main campaigns, or you can work your way up the ranks via online Xbox live multiplayer. Needless to say, Battlefield is an awesome game, but it's just that... A game. Halo is a complete world with numerous opportunities for any gamer.
P.S. Not to mention the Halo books!