I do own both of them. I usually only drive them on sunny days.
I have owned the M535i for 3.5 years. Bought it without seeing it in person. Never driven one or been in one before. Instantly fell in love with the M30 engine. Sounds fabulous, like a proper classic straight 6 should with plenty of low end torque. I have had lots of friends who have been as passengers in it and commented on the smoothness. I have upgraded a few things like suspension, wheels, brake lines. I do worry at times that it might not brake as good as the modern cars and I might be in trouble one day hence E34 brakes are on the cards. I do drive it hard from time to time and these cars love it. I keep all the original parts.
I bought the E30 Mtech1 four months ago. The initial drive was disappointing thanks to the M20. But the whole car feels very light and compact in comparison. The seating is lower, you feel more tucked in. It's getting a few things done atm - injectors cleaned, valve timing, new heater hoses, suspension, wheels, brakes... Pretty much the same as the M535i. The stock E30 brakes are worse than the E28 stock ones.
Rust is the biggest problem. No classic BMW is rust free. They either have rust or been repaired. Oil leaks is the second. But these cars are fairly easy to work on. I don't have much mechanical experience, so I rely on the forums and YouTube for help. But I do most of the maintenance myself.
Price wise, $15k should be enough to get you a decent one with some minor issues. The M535i are selling for $30-40k in Australia these days.