For me, it depends on what you have planned in MAD, and when.
Arriving Madrid at 8 am can be a terrible shock to the body - especially if you can't get into your hotel room and have to spend the morning walking around Madrid like a zombie.
Arriving in a European city in the evening is my preferred timing, especially if I could get a long enough layover in MUC or FRA for a shower in the Senator lounge. I did this in MAD in May, and it worked out splendidly, got seats on the Metro into the city center (not likely with an 8 am arrival), and had just enough time for a late dinner (which isn't late by Spanish standards) and then off to bed - slept wonderfully and was raring to go the following day.