If you’re interested in developing a game these days, particularly a VR game, there are a few different options. But two of the best in terms of popularity, flexibility, and breadth of features for indies and large studios alike, are undoubtedly either Unity or the Unreal Engine from Epic. While the rivalry between the two may not be as pronounced as the console wars or VR wars we see debated everyday, they’re undoubtedly competing for the same customers.

During GDC, I sat down for a private interview with Epic’s founder and mastermind behind the Unreal Engine, Tim Sweeney, and he outlined in straightforward terms why he thinks more developers seem to be using the competition’s software. Unity’s CEO John Riccitiello recently said about 1.1 million people actively use Unity each month and Sweeney told UploadVR “we’re at about a quarter of that size with Unreal.”

To Sweeney, though, that’s not the important measure:

There are different ways of measuring reach. If you measure it by the revenue of actual shipped games I’d imagine that Unreal is ahead of Unity. . .We’ve identified seven franchises that have grossed over a $1 billion each and they were all created with Unreal from teams throughout the industry, which highlights the scale of development with Unreal.

Throughout our conversation, it was clear that the most important aspect of the Unreal Engine was, without a doubt, how powerful it could be and the vast suite of features available.

One of the best things about Unreal is how well it scales up…it can be used to create an indie game made by a single person in a matter of months or a development studio of hundreds of people making a game over the course of several years. It’s optimized to be the most practical way to ship a great game.

And of course when he says “ship” in the context of 2016 he means the act of finalizing, packaging, and supporting a finished game product. That ability – particularly in terms of post-launch support, bug fixes, and patches – is often complicated and frustrating for developers. That entire game production and development cycle is a big focus for Unreal – not just getting people started.

It’s really a two horse race at the moment if you boil it down to what people are actually using. When you rank the engines by ease of use to get your first lines of code up and running, we’re certainly in second place [behind Unity.] A lot of people use Unity because it’s so approachable. The less hours it takes to make something, the quicker you can build a simple experience. But the really hard part of building a game is shipping it. If you measure the number of started games versus the number of shipped games it’s probably 100 to 1. We really focus, as a game developer and engine developer, on the hard problems of shipping a game and that’s why we give out the full source code. Every game is being developed a different way and experiencing different bugs and every game developer has the power to fix their problems on their own.

Unreal recently revealed a VR editor within the Unreal Engine that allows users to quickly and easily edit environments while immersed in VR. This was a huge move for VR game development because of the interactive and immersive nature of VR. However, Unity is also creating an elegant solution for VR content creation as well. Only time will tell which engine winds up as the preferred medium for VR game development, but with each platform squarely focused on different priorities, there may be room for each to shine.