Depends how much stuff/equipment I plan to bring. For me, it's often more a question of flying versus the train. Driving in western Europe is expensive, with fuel (1,67 €//litre SP95 E10 as of this writing) and road tolls quickly adding up. Flights can get expensive on short notice, and I generally only plan week-long vacations like spring and fall holidays about 4-6 weeks in advance. Plus, our gouvernements actively dissuade us from flying because something about greenwashing and COČ emissions, so plane tickets are inherently expensive unless you're flying from a far-out aeroport with a ULCC.

So in short :

• how much stuff am I carrying ?
• how far in advance/or how late can i plan (ticket price) ?
• Cost of hiring rental car at destination if arriving by train or air
• do I need to make stops in cities/places along the way ?

But generally as a solo traveller, destinations more than 5h away, I start looking at car + train or flying. Less than 5h, I lean towards driving.