Thanks alot for your answers!
Severe accidents due to being unfamiliar with the type seems pretty rare with planes that are easier to fly (p28/c172), my teacher thinks that i wouldn't have any problems as long as i'm very strict with the fueltank and the different nosewheel-steering, but he even thinks that i wouldn't have any problems jumping to the cessna 150 without any check-out, he has roughly 20,000 logged hours of general aviation so he might not have any bigger problems with it, the fact that he claims going to the c150 once mastering the c172 makes me a little bit un-confident taking his words even tough he is a very safety-minded pilot and instructor, so i needed a little bit of advices from alternative sources.
Well i'll start flying both then, maybe could it make teach me more about flying than sticking to the same type