Round trip vs two one-way tickets?

There's generally only one major disadvantage to booking two one-way tickets in situations like this - Change/Cancellation fees!

If for some reason you have to cancel your trip, instead of paying a single cancellation fee you'll have to pay two - one to each airline - and odds are that each fee will be the same as the single fee would have been so in effect you'd end up paying twice as much.

There's a number of other minor disadvantages, such as the possibility of check-in taking longer as you'll potentially need to show proof of your return ticket (I don't believe this is required for Mexico, but it will be for other countries) as the airline won't have the details of your return trip. You may also have additional questions at immigration/customs - but these will be easily answered - Just make sure that you are carrying details of your return flight with you!

A single ticket can also be advantageous in the event of delays. eg, if your outbound flight was delayed by a day or two (eg, due to weather) then the airline will normally be fairly flexible in allowing you to delay the return by 2 days as well to keep the total length of your trip the same. Obviously if you've got 2 one-way tickets this isn't going to happen.

Many travel websites will allow you to book one itinerary with flights from different carriers on it, so you could at least book both flights at once - but keep in mind that many times these will simply be booked as two separate tickets so other than only having to enter your credit card number once you're not gaining much over purchasing the two one-way tickets separately.

Realizing this thread is 3 years old, I just wanted to add a difficult experience I had.

Booked RT airfair for my family and myself. We all boarded the departure flight, but our passes did not scan. Instead the ramp agent was collecting the slips from everyone.

At our destination, half way through the trip I checked on our return flight info to see that my seat had been cancelled. I was told over and over by the airline that their policy on RT tickets is to cancel the return one when a person fails to take their departure flight. I took a lot of calls for me to get through to them that I was on the plane (though they still doubted it) and to restore my flight home. Great way to ruin the relaxing benefits of a vacation.

I now book all my airfare separate unless there is a connecting flight. (Which I avoid as this same airline has lost my luggage twice due to connecting flights at BWI.)

two one ways are definitely becoming more and more the choice to take! With online search engines and direct booking with airlines, it is now possible to re-arrange things on a moment's notice.

But if you bought a return-trip and happen to miss the first leg you are likely to lose the return flight, simply due to the airlines' stiff "no show" rules. They do not recognize that severe weather might have been preventing you from reaching the start airport. They do not care that you had a whole bunch of other things on your mind at the time. They just point to your not-telling them in advance. Ergo, the whole trip gets cancelled.

This is the most stupid rule ever. Any other service sector that denies rendering services you already paid for (the return trip) on grounds that you failed to use the first half of purchased services???