the 528 has a reliable engine. I think it was engine of the year.
outside of that i think you have to consider a few things. 535 is a brand new car, will probably have issues. Might be in the shop more then you would like it to be.
it should not cost you $7000 a year to maintain the 528. as long as you take care of it. preventative maintenance is key.
if I were you i would keep the 528 for a few more years. take the extra cash not spent on a new lease and put it towards a larger investment. I think it would have to come down to whether or not you want a new car as opposed to an older one.
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