FWIW it is simpler because BUSL source can change to any license (including copyleft), whereas FSL can only change into MIT or Apache 2.0. So it goes from N possibilities to 2.

> Is there any functional difference between apache and MIT?

Apache 2.0 has an explicit patents clause, whereas MIT does not.

Better explanation here: https://opensource.com/article/18/2/apache-2-patent-license

> Is there any functional difference between apache and MIT?

We only use FSL to Apache 2 so we would have been happy with just having this. However the issue of lack of GPL 2 compatibility of Apache 2 is known to us so having a GPL 2 compatible option with the MIT license we believe makes sense. In practical terms if you want to adopt FSL from scratch and you have no GPL 2 concerns, Apache 2 is most likely the better choice as it also carries a patent clause.

To add to the sibling comment, the Apache license requires you to carry along the NOTICE file, whereas MIT does not. This is often overlooked.