I’ve never driven a mk3, but the general consensus is that they’re more liveable than earlier cars. Ignore any ‘comedians’ calling them boats. They’re barely any heavier than Mk1s or 2s. I think that the engines in a mk3 are more prone to failure though. On the flipside, you’ll get a newer car without as much rust to contend with.
I personally don’t like the idea of buying a car as an investment, but Mk1 prices are rising fairly quickly without any sign of slowing down. Even rusty MOT failures are starting to make actual money now, so a solid one might make you a profit when you come to sell. Mk3s don’t quite seem to have reached the bottom of their depreciation curve yet. They’re still comparatively quite a modern car.
Take a look at both and see which you prefer before commiting to buying either. In many ways they’re very similar cars, but depending on what you’re planning on using it for both have their benefits.