This past April, I went to
Disneyland. This is my experience with the flights. I'll let you decide if booking round trip or single flights is better.
I originally booked round trip on Delta. However, they changed my return flight by 7 hours. The flight out was still a nonstop at a great time for me, so I wanted to keep that flight and cancel the return leg and rebook on another airline. Not possible. I had to cancel the entire round trip, and rebook the flight out as a one way.
I then booked the return flight with United and was actually able to get even more time in the park on the last day. Originally, I was supposed to fly out at 6:30pm; Delta changed it to 11:30am. On United, I found a nonstop that left at about 1:00am the next morning. I actually was able to see World of Color on my last night.
After booking all this, I decided to tack on a trip to New York City at the front end of my Disneyland trip. Long story, but yes I was going to fly from New Orleans to NYC, then nonstop to LA for DL and then back from LAX to New Orleans. I couldn't book round trip for that because I wasn't going round trip, and I already had some of the flights booked so I couldn't book a 3 leg trip either. I had to book singles. I booked JetBlue from MSY (New Orleans) to JFK and then JetBlue from JFK to LAX.
So....up until the night before my flight, I had this booked (all single legs):
MSY - JFK on JetBlue
JFK - LAX on JetBlue
MSY- LAX on Delta (hadn't yet canceled)
LAX - MSY on United
Just as I was going online to cancel my Delta flight and pay the fee, I got an email from JetBlue. It was the day that Boston was locked down due to the marathon bombing suspect search, and my JB plane was coming in from Boston-Logan and my flight out the next morning was canceled since the plane didn't leave Boston.
I called up JetBlue; they had rebooked me for the flight the following morning 24 hours later, but since I was only going to be in NYC a few days anyways, I didn't want to fly for such a short time since I was also flying all the way to Disneyland. JetBlue was fantastic. They canceled both my flights for free, even though they were not round trip and they were likely under no obligation to cancel the JFK-LAX leg of my trip since it was not linked to the affected flight and it was not round trip. And they did all this on a night when the call center was dealing with flight issues from severe weather in the Chicago area and the crazy situation in Boston.
In summary:
-Round trip flight was a little annoying when one flight was changed substantially. It would have been a bigger problem if the flight I had wanted to keep had gone up in price when I had to rebook.
-Single flights could have been an issue when one flight was changed substantially and I needed to cancel both.
-Flying different airlines down/back was not an issue on the actual trip.