When you want to assign the position of an object you cant use float you need to use Vector3 and when you want to access a specific axis(y,x,z) is float, then, it is preferable to put <= than == because == is equals so you are asking for a specific number so if your player is falling to get the specific number is a little difficult and <= is less or equal to.. so this way would be better.
Also if you want you can put an empty respawn object and just create a variable:
var respawn : Transform;
then when he falls send the player to the empty object: