witch oscilloscope to buy

I want to buy a scope and need to know how quick it must be. The Arduino runs on 16 MHz. 5MHz Pc Oscilloscope, 10MHz Pc Oscilloscope or 25MHz Pc Oscilloscope. (PicoScope) The prize dif is more then 150 euro's.

I wil use it to measure inputsignals from e.g. 433MHz reciever.

Depends what you want to measure. The analog bandwidth determines the frequency at which the measured signal is just 0.7 of the true signal. So with a 10MHz analog scope you have a voltage error of 30% at 10MHz sinus signal. This gets worse for higher frequencies. Maybe you'll see 15MHz, but the amplitude is definitely wrong. If you want to measure say square waves this is even worse as you need "overtones" to get the shape right. As a rule of thumb: if you need decent reproduction of X MHz, you need 5x X MHz of analog bandwidth. With digital scopes (usb ones /DSOs) you also need to look at the sampling rate. Decent devices have sampling rates of 10x the analog bandwidth. So for 10MHz you need at least 100Ms/s (real time sampling rate, anything else is junk).