In March I would suggest that you choose the south of France option. Generally, as you head south the weather will get better. The danger with going to Germany at that time of year is that the fronts are moving in the same direction (west to east) as you. If you fly east you could be heading towards a front that has already passed the UK. More importantly, however, is the return trip. The problem is that when its clear in Germany for your departure new fronts could be heading into the UK from the west and you may have to stop in France or Belgium and wait for the front to pass and then continue your trip. Heading south avoids a lot of these problems as you are generally at similar longitudes to the UK so if there is no big front in France it is more likely that there won't be frontal activity in the UK. Even if there is one in the UK it might pass as you are flying north up through France on your return trip.