Halo MP will always hold up. You can still play Halo 3 with no problems at all and if they hadn't shut down the servers people would still be playing Halo 2 (you still can using one of the free services).
MP is great and will be lively for a long time. If you like the normal size and scope of BF3's maps then I suggest starting in Big Team Battle or Dominion and seeing how you like it. They're focused on the bigger maps with an emphasis on larger-scale battles with vehicles involved. But if you like the smaller BF3 maps that are infantry-focused, then start with regular Infinity Slayer.
I'm actually thinking about going back to BF3. I haven't played since a little bit after Karkand came out so I have a buncha maps and weapons to catch up on (I think). I really did like BF3 more than COD.
Anyway, I do like Halo and there are a good bit of gametypes which they rotate occasionally. There aren't Big team Objective game yet and not too many Big team maps unless you buy the Map pack for another couple. Not sure about that though because I don't love the new maps, hence why I may go back to BF3.
If you are in to smaller 4v4 matches there are different maps in that rotation and more game types.
There aren't nearly as many weapons, and one trumps the others in most situations so you end up using it mostly.
If you have a lot of Friends that play there is an unlimited amount of user maps you can play privately.
Vehicles are kind of similar, in that they often provide insta-kills for non-vehicle players. But otherwise killing enemies takes a lot longer. You need to put 5 shots from the DMR or BR (the standard precision weapons) into an enemy (as long as the 5th is a headshot that is) to get the kill.These weapons aren't like your FAMAS or M16 from BF3, they're a lot slower firing and are only semi-automatic. There's plenty of 1 hit kill weapons mind you, but they're all power-weapons and you can't spawn with them, you're either rewarded with them for playing well or they're placed on the map as weapon drops (and usually points of focus).
Also, because it's harder to get the kill, you'll find yourself dying a lot at first, so it's very easy for new players to get disillusioned with the game and give up. The learning curve is fairly steep.
Fire Bucket http://img.photobucket.com/albums/v413/N_o_R/ron.gif http://img.photobucket.com/albums/v413/N_o_R/BtJ6F.gif