I drive the E90 M3 for 7 months out of the year and park the X3. the X3 is also good for Sampson our Vizsla to ride in. I draw the line on having the dog ride in the M3. Besides he would likely look like the dog in the BMW M5 add where all you see is the dogs head and paws sticking out from the seam of the rear seat. Love that dogs expression.
I do think that bone stock clean E30 M3's will continue to raise steadily in value. There were less than 5000 brought to the US and many have been ravaged crashed and driven into a state of costly repair to bring them back.