There are no hard and fast rules on the cheapest days to fly, but as someone else said Fri - Sun are usually more (many airlines have a weekend supplement too) Monday is often higher too because business flyers tend to travel that day hence more seats are sold and the remaining seats get their price hiked.
In general airlines work on the premise that the more seats they have left to sell, the lower the price until the very last minute when they hike the price up. In general, Tue - Thur are the cheapest days to fly but that isn't aI lways the case.
Everyone will have their own views on villa vs. hotel so all I can do is give you mine which is of no more or less value than anyone elses!
Having a villa is like a home from home situation to me. You're still cooking, cleaning and doing all the things you'd do at home, you're just doing it in the sunshine and after a tiring day in the parks! A villa comes with additional costs too. It may be cheaper to rent than a Disney hotel, but when you factor in food, parking fees, petrol etc. You won't be saving as much as you might think. Having said that, it's good for people who like their own space and don't want to LIVE Disney 24/7 for 2 weeks.
Disney hotels offer you the free dining option, you don't pay to park, you can limit the use of your hire car and therefore your petrol costs. I think the best thing to do if you don't mind either option is to just do the maths with all things considered. Get an idea of how much food will cost for your party, add on the parking and a tank of petrol and see what the numbers look like.