better in terms of the fitness gains to be expected
With a smart trainer, you can quantify your fitness gains in a way that I don't think you can without one. You could say "I've been winning more Zwift races" or "I feel stronger," but you couldn't put a number on it. If you're the kind of person who is motivated by chasing numbers, that's an advantage. A smart trainer also lets you be more precise about training: FTP-oriented intervals have a high end of 105% FTP; VO2-oriented intervals have a low end of 110% FTP. I don't think there would be another way to train at such close power levels reliably.
You can absolutely get fitter riding a "dumb" trainer, but I think the gains will take longer and will be less focused.
If you haven't seen
DC Rainmaker's smart trainer buyer's guide, check it out. He also has one for smart-trainer apps. It winds up being a deep rabbit hole.
I've got a wheel-off smart trainer (Cycleops Hammer) and a sacrificial bike that lives on it full-time. I think that whole setup cost me about $900, but I was buying the trainer as NOS and got a really good deal on it. If I were starting from scratch, I'd do the same again. I think wheel-off trainers will be quieter, probably more stable, and obviously you don't need to bother with a special trainer tire.