apple watch SE vs series 5 (or series 6)

I have the original apple watch and I am looking for an upgrade. I live in Sweden and I am looking at the 44 mm SE (ca $392), the series 5 (ca $476) and the series 6 (ca $547). Which ever I choose is going to better than my present watch, but what I want to know is how much really necessary extras do I actually get for the extra ca $80 or $150? I know the techs and the bells and whistles but is the extra $80 worth it just to get a screen that does not switch off and the ECG function? Or the additional $70 to get the blood oxygen meter?