Last time I looked the extended hours were only showing maybe till 11:30 pm versus the 10 pm time. Staying till 1am would be great if they still offer that.
It depends on what time the park closes to the general public. This perk is called Extra Magic Hours. In the evening, it means the park is open 3 hours later than the normal closing time. They also do Extra Magic Hours in the mornings, where the park opens one hour earlier for resort guests.
Even when we stayed on site, we did not use the Extra Magic Hours. In general, the parks that have Extra Magic Hours will be the most crowded parks that day. So if you're just looking to minimize crowds, you're better off choosing a non-EMH park each day.
That doesn't mean it's not a nice perk for some people. I love being in the parks after dark and I'd probably take advantage of some evening Extra Magic Hours, but we've always had young kids who couldn't make it past about 8 or 9pm. And I'm not getting up an hour earlier to use the morning EMH, so for us, it worked best to pick a non-EMH park and try to get there at the normal opening time (usually 9am). The first two hours of the day are the least crowded, no matter what day it is or what time of year.
By the way, if money is no object or you're budget is big, then by all means, stay on-site at a monorail resort (Grand Floridian, Polynesian, Contemporary). Being able to walk out of your room, jump on the monorail, and be at Magic Kingdom, is a huge convenience. If you're choosing between off-site and the Grand Floridian, there's no contest.
But if you're on a budget, like many of us, you might not be looking to spend $250 and up per night. If you're comparing off-site to staying at a value resort (All Stars, Pop Century), then the value resort doesn't have quite as much of an advantage.