Search and Destroy has a massive difference on whether it is Core or Hardcore. I'm not very good at Core, but I can tell you a bit about Hardcore:
 

• Perks and equipment are entirely up to you. Obviously whilst I'm helping you, I can't judge your playstyle, your accuracy, etc. I play with the pre-built class 'First Recon' but I know a dude who plays an ACR with heartbeat sensor and ACOG.
• Unlike some of the game, Hardcore Search and Destroy can easily have some good tactics on it. It's all about finding out what your enemy is doing, and getting rid of it. Marking out key areas of crossfire is a great way to play, and it's even better when you know when the crossfire normally occurs (for example, in Skidrow, down the tunnel there's normally an enemy or two who pop up round there)
• Construct a few routes of your own, which either utilise areas of crossfire, or do the exact opposite
• When I say the exact opposite, a great way of dispatching enemies is by outflanking them. Getting behind an enemy is an easy way to get rid of them-it'll only be around halfway through each round before they'll watch their back.
• With that in mind, be sure to watch your own.
• If you are killed, chances are the enemy will follow that same path. If you have a cry for vengeance, you can attack, but be weary. The enemy may know this and camp at an area where you'll be an easy target. A good idea as to make a perimeter around that area, so you can find him, like a fox hunting a rabbit.

• stealth>power, since IW made it so KS can be used you want to stay undetected.
• the game mode works best when you work with a team of people that you've played a lot with. 
• Team work is key, so that you don't get flanked. 
• Learn the maps and shortcuts through them. 
• Hardcore is more difficult (if you're seen, you're pretty much dead). 
• Use an AR or SMG
• Planting the bomb = Higher chance of success
• When defending don't push too hard.
• Camping while defending is a more legitimate strategy in S&D than in any other game mode (it's useless in other modes unless you are a kill whore).
• Camping on attack is the worst thing you can do.

well which is better core or hardcore, and do you still get the same amount of points, because i know that in core you get like 1100 or more for a headshot, which is awesome.

" well which is better core or hardcore, and do you still get the same amount of points, because i know that in core you get like 1100 or more for a headshot, which is awesome. "

Camp ftw.

Personally, I prefer playing Core in any game mode. The XP earned from doing things is the same, but in Hardcore you may not get as many opportunities to earn points. At the same time though, you can use a wider array of Perks to exterminate your enemies. If I were you'd I would stick to Search and Destroy in the play style you prefer. I'm more of a Core player, but here are some tips, presented in a table for some reason:
 

I use the AA12 as my sidearm in SnD, and yet everyone says use the SPAS-12. is there any reason why I should switch? It seems to me like the aa12 is better in every single way (apart from ammo, but rounds never last long enough for me to run out of ammo anyway). 
 
 Also, as for my tip- be crouching a lot. Crouch-walk round corners if you're not sure what's round there. It makes your footsteps quieter (if you're not already using ninja) your shots more accurate and your aim more stable.

i think that the AA12 and the SPAS-12 are almost equal. The AA-12 has the automatic and much quicker, while the SPAS-12 has much greater damage, but you have to reload.

Tip: Don't play Search & Destroy

SnD is a crapfest full of noobtubes. Fuck it.