What Sensor to use ultrasonic or...

Ok.So pretty much i Had a project in my mind and recently i decided to make it .But there is a LITTLE problem... Before talking about the problem i probably should tell you about the project...
yea... So pretty much this project is system which consists of 3 major parts.

I.the INPUT(Sensors)
The sensors are put and are giving a boolean(T,F)
Either there is a car in the parking spot(T)
Or there is no car there(F).

II.the COLLECTOR(The computer type thing)
Collectes all the Input information and sends it to the server.

III.the END(Server and Application)
After the information is sent to the server, server processes it and sends it to the application or website.

Thats pretty much it...

And now about the problem
Sensors...I dont know which one to use.
For Example`
Ultrasonic is great solution,but mother nature will f#$% it up.
Camera is a great solution too,but is not working stable with arduino.

What do you suggest?

So if I understand your message correctly, it can be summarised to "how do I detect whether a car is in a parking spot or not?", right?

One option: place an induction loop on/in the ground. It'll detect that hunk of metal in any weather conditions.