Welcome to the SeeMeCNC Community

Still looking on what printer to buy... V2.. probably..
looking at parts that were printed, why are some so smooth compared to an other part about the same size..

does-it have to do with the material used..?

Depends what you mean by smooth, but general quality is dependent on a lot of things
In no particular order

Slicer settings - Layer Height, Temperature, Speed, etc etc
Material
Printer Calibration
Quality of mechanical construction

Very few printers today are really plug and play, there is a lot of messing around to get to great prints.