Only you can decide if the crowds will ruin your experience, but I have to say that Wishes and the show that pecedes it are fabulous!!! WE are going in December with my DS and his wife. DS has not been back to Disney since we took him when he was 12, and so this is a huge deal. He is not good in crowds. WHile I am going to plan as well as possible to minimize his having to deal with crowds, we also need to consider that in December the Christmas parties make it hard to avoid a crowded MK if you want to see Wishes on a non party night. If I cannot figure out a decent plan, he is just going to stay behind that night with whoever else wants to veg by the pool.
Illuminations is a better bet for lower crowds, but you also have to be ready for those folks who let you all stake out the perfect spot and then decide your space is preferable than theirs and they move on in. The truth is that no matter how carefully you plan, there will be people who to to encroach on your personal space because they want your spot without haveing had to wait for it.