Survival map and the normal map is the same.
Normal map has Boss spawns after every 20 monsters, and starts all over (depends on the Waves you are playing it will be after every wave)
Survival map is the same but the monsters do not stop, one continuous spawn and stops when you can't kill them in time.
Survival has a different tactic on placing the correct towers in time and upgrading as fast as you can.
Gear plays the other big part, bad gear will not help to get a good score.
If you need the toilet - hard luck (just joking - just press "p" to pause the game)
Normal - you have ample time to change, break down towers between spawns.
Survival - you have to change, break down towers while monsters keeps on spawning.
But its fun when you have the correct gear.