With UE4 you can pay for a single month and retain full access to the tools you've paid for, you just don't get updates. What you could decide on doing is paying every few months when an update comes along that offers something you need / want.

UE has UDK, which allows you to use their scripting interface and access some engine classes, to but no access to most of the source code. It doesn't unlock as much a proportion of power as Unity Free out of Unity Pro, but lets you get a taste for the asset pipeline, workflow, level editor, etc to decide if you want to go for UE.

Unity's paid version doesn't even have access to source code -- which would actually be very helpful for many devs because Unity's buggy as heck and have very long bugfix turnaround times, that we'd be happy to dive in and try to fix for ourselves if we could.

Not to mention more flexibility/modification for the dev's uses of course.