Seriously unless you think your children will want it when they grow up, don't keep it. It's there for a purpose but do you really want to carry it with you through house moves and growing older - it'll only depress you in five years if you can't fit into it.
Keep it!! Mine's stuffed in DD's dressing up cupboard, complete with footprint where Dh stood on it during our first dance. I can still fit into mine 15 years later <preen> and it's just part of our family history now.
Just had a quick look on ebay, there isnt a single Maggie Sottero 2nd hand dress that sold more more than £100, most of them will have been £1000-£1500 new... £100 isnt going to get you much of a break away.
It depends on whether you need the money though, and if you are going to sell it then do it now, as it will sell better its current season.
Oh and to get it dry cleaned will probably cost you an arm and a leg.