Similar ideas. The MoH glacier is wider and you start of more spaced out so you can really get up some speed and properly ride down the glacier. The final third of the MoH course is down a walkers trail that you aren't allowed to pre-ride before the race. MoH is a shorter course but is greater vertical drop (ie. steeper). Both are good fun races. MoH seemed better organised although I guess there's a few less people to manage.
Alpe d'Huez seemed better for riding. LDA might be ok for a few days but would be good if you could split your time between both places!
