I have done both. Most recently, laundry. I got up one morning at 6:30am foolishly thinking I was the only one who would want to do laundry at that hour. I was wrong. All the machines were full! I didn't have to wait long--about 45 minutes. So I showered and ran back down. Now, this was a "late" morning for us. We weren't leaving for the parks until 9am. Our room was also close to the laundry facilities. I was able to get everything washed and dried in time but it was a hassel. The machines remained full the whole time I was washing/drying.
If you have an off day and can keep checking for free machines, I say go for it. Otherwise, I say overpack. I thought one night I would do laundry but by the time we got back to the room and put the kids to bed, I was exhausted and the last thing I wanted to deal with was laundry. With Magical Express, it's not really a hassel to bring that extra bag overpacking sometimes incur. It's $2/load to wash, and $2/load to dry. I am sure you ahve read that is usually takes two cycles to dry the clothes or split them into two smaller loads to dry. Well, it's true! Our darks (with jeans) took two cycles. Glad I had the extra quarters.
I did pack a small pop up hamper which was great for keeping clothes off the floor without having to keep a suitcase out. I have heard some bring two--one for clothes and the other for toys (if you have little ones). Good luck. Have fun overpacking!
HTH, Blakely