I came to do this experiment from an incident at an escapees escapade at Stockton, CA. Mimi and Johna were parked near and parallel to our coach with their cabs pointing essentially west. Mimi got up on their coach and tilted their panels up. We were all standing around talking one evening and I observed that the sun had moved around so that it was behind their tilted panels while our panels, being flat, were still making electricity. The question became did their panels make enough electricity in the middle of the day to overcome the fact that they quit in the late afternoon?
The answer is, of course, that tilting in in mid-winter will make more amp hours. However, flat panels work as well or even better from mid-spring to mid-fall. There is another very subtle issue about the tilted panels. If you don't have enough battery capacity to store their big mid-day output, then tilting the panels, even in mid-winter, may not be a good deal because the tilted panels stop making electricity earlier in the day. You, therefore, will draw electricity from the batteries for more hours in the day. In that case, you may be better off with flat panels if they will mostly charge your batteries because they will supply electricity earlier and later in the day. Solar power has so many crazy interconnected relationships like that, that it's very hard to give sound advice. Every answer starts with, "well it depends", and/or "well, that's complicated".
Linley