I would definitely recommend staying on site. If you are worried about price, look at the value resorts. They don't cost any more than staying in a Holiday Inn in Kissimee, but they are still magical at least. The only "non-Disney" hotel I will stay in is Shades of Green, and that is only if I travel there with my family because you have to be military to get in there. My dad is retired Air Force. Of course, with military comes discounts. I will do that. But even there, you can walk to the TTC in a few minutes from there. Plus they have a fairly regular bus to the TTC. I can't speak for other "off site" hotels, but I can't imagine transportation being any better.
WDW transportation is great. My guess is that if buses don't come as often as stated there is a reason. Perhaps someone just missed the bus when they got out there. That happens. It takes a while for the next one. Perhaps someone who was disabled was getting on/off one. That can delay them as well. Sometimes it is best to know what ALL of the bus options are. Say there is a bus to each park, and also one to TTC. If the park buses are 40 minutes apart, but the TTC bus comes every 10 minutes, it may be better to go to the TTC and catch the monorail or another bus to the park. Sometimes this may be quicker. Other times it may take longer. But mentally you may feel more productive taking longer while "traveling" than if you are sitting for 40 minutes waiting on the bus you just missed, without knowing when you missed it!