Yes, it sounds like a strange but much important question. If not, I will probably not write this article.
Let's start. The question here, as the title says, is to find out whether we want to make a round trip with one airline or do we prefer to go with one and return with another company. Again, the answer is not clear but there are many things that can be said to give you a little guidance.
One might think that if you can find a good price with an airline to go but the return price is too high, the efficiency will be less expensive. Well, it could be, but rare. If the price of air tickets with the outbound airline is high on the day you return, it may mean that efficiency will also get you high on this day. The market is the same, and since the reason for the high fare is the high demand, it would be strange if two airlines covering the same route on the same day had different demands, right?
Now that I've agreed, I will say this is not always like this, and the exception comes when we talk about terrible timing flights. I call them that way because they really are. You need to wake up at 3:00 AM to reach the airport at 5:00 AM because your flight leaves at 7:00 AM. Well, you might have found a good airline ticket here, if you're not flying at the same time, of course!
A general rule also applies. Whatever path you choose, hot deals will almost always be for round trips, not for single trips. This puts you in trouble if you can find a good offer with another airline. If you are traveling with one and are back with the other, you cannot get the deal but at the same time the return price with the other airline has been very low! It is not easy, but if an airline has a good list of discounted return and return deals, it is almost certainly the best option that you can make is those deals.
At the same time, traveling with one airline will allow you to accumulate miles if you join one of their frequent flight programs. If you go with one and come back with the other, you will always end up getting points with all airlines but none of them will be replaced by anything because the quantity will not be high enough. However, actual alliances make you able to change a few points from here to there, so you should think about this option as well.
After all, I would like to say that you will usually find the best deal to fly with the same airline from all points of view, and I only recommend that you travel with another company in the event that the flight conditions provide clear advantages to you. In this case, keep in mind that you are sacrificing miles for a slightly better price, so think carefully about it before making a decision.