Essentially you lose everything. It's like starting over again from level 1.
This time around you can pay Prestige Tokens to get early unlocks, (ie, the AK-47 or Steady Aim at level 1) but the weapon/perk/gear's status is reset and you will have to level it as you normally would on each Prestige. For instance, if you had gold camo on a gun and all the attachments, even if you use a prestige token to "Unlock Gear," you'll lose the attachments/camo and you'll have to level the weapon again.
The only thing that is saved from one prestige to the next is any Titles and Emblems you've received from doing challenges. The challenges themselves reset, however. If you're really close to the end of a challenge that unlocks a title or emblem, it might make sense to finish it before using your Prestige.
You also get to keep the quantity of custom classes, although your custom classes reset to the defaults when you Prestige. You'll also not have access to custom classes until you reach the level where they unlock on the first pass.
Survival mode never ends. I have found the best strategy thus far is pick an area you can defend easily next to 1-2 buy points, preferably ammo and grenades crates.
It needs to be an area with overhead cover to avoid choppers, ideally should only have 2 entrances at most. I have yet to find a good spot on maps that have 1 entry point.
Get the sentry granade launcher, have your partner do the same, Get Riot Squad reinforcements,
The idea is you can stay behind the sentry guns and riot troops and kill everything, You have Sentries guarding 1 entrance and the riot troops at another.
You will need explosives later on to take out juggernauts, normal enemies will start to take a whole clip to kill around lvl 30 if you dont got for headshots.
C4 and Claymores are helpful but can get a little expensive to buy every round.
Flashbangs are invaluable for taking the juggernauts out. By mid lvl 30's you facing 4 juggernauts, and exploding dogs with armor on most maps. It can get crazy. I have not found any strategy of displacing to be effective, the enemy troops will claymore most doorways and choke points.
Perks - Longer Sprint was the most helpful I found.
Best Answer
At the lower levels, you're more likely to achieve bonuses like "killing spree" for killing a number of enemies without getting hit, or "rampage" for killing many enemies in quick succession. The enemies also go down easier.
At the higher levels, you more than likely won't be getting as many bonus points, but the enemies are all worth more. However, the levels become far more challenging and the waves take longer to clear.
I mention "round 15" because that's the minimum round you have to play to on every Spec Ops: Survival map in order to unlock an achievement. There's nothing magic about this round otherwise.
If you're in this for experience gain or for the achievements, there's really little benefit in forcing yourself to play the higher levels, or being particularly disappointed if you fail. It can be good practice or a fun challenge, but I wouldn't feel pressure to survive every round unless you're trying to beat your (or your friends') high scores.