Bit of both depending on mood and aims. I will often set off with a route in mind but then change or adapt plans along the way as the whim takes me. Occasionally plan a gps route and then stick to it religiously.
Don't think I have ever set off without an inkling of an idea of where I will head, how can you?
I'll often set off without an inkling where I'm going, turn left or right out the driveway.
But then I've lived and cycled within the same patch for 30 years, and it's all very good cycling country (if you like hills) occasionally I will come across a track or lane I don't know, but I will have a reasonable idea in my head where it must end up eventually.
It's my idea of a 'recreational cycle ride' otherwise I'm going somewhere for a purpose, but still I'll prefer to go out one way, and back another if I can.
If I'm out for days or weeks, in an area I don't know so well, then I will start with a rough route, then usually deviate, as the whim, and the map takes me - that's the joy of touring with a tent