The games are similar, yes, just like all games are similar to another. Just like your favourite song probably sounds just like another song.
In my opinion, the biggest separation from the 2 games, is the shield. The shield in Halo vs no shield in CoD is what makes the games different to me. One shot can kill you with almost any weapon in CoD, where as 4 (well placed) shots will get you a kill in Halo, thus creating fantastic arena gameplay
This goes for all Halo games, of course since the shield has always been around.