is say they are about the same with only minor problems on both, the m30 is definatly more capable at hauling the semi lardass e34 around but the m20 gets the job done, ether will go 300k miles with regular mantance but you are not working the m30 as hard, both can want some head work above 200k depending on mantance done, the cooling system is a little weak on the m30 and it wouldn't be a bad idea to factor in a new radiator/t-stat/water pump for anything you get as prevenitive insurance because i've seen many a motor killed due to a small part failure