If it is reasonably priced and in pretty good shape, go for the SD. There is no comparison between the SD and the 240D. You are light years ahead. That said both are about the most reliable vehicles ever made.

Also window problem on the SD is likely the sliding jaw; very common and very easy to fix.

From a mechanic's standpoint the SD is a dream to work on; I have no experience with the 240D but would expect the same.