400w leapton or 420w Jinko for +500 euro ???

If the question is, which panel is better, the answer is "there's generally no difference between makes".

So then it becomes a question is 400watts extra worth €500? Normally suppliers will add a 400w extra panel for €250. so on that grounds i would say it's not great value, but if the overal quote for everything is decent then I'd be inclinded to go for it. I know that's sort of sitting on the fence, but I can tell you that the biggest regret around here is that people didn't get more/bigger panels on the roof in the initial install.

Aside: I assume your coupling that to a 6kw inverter?

Nahh no real difference between any of the make/models of panels. You might get one manufacturer who'll (after 10 years) claim to give you 88% production, and another might be 85% after 12 years etc, but generally they are all much to a muchness. Expect to get 20-25 years out of panels and at that most of them will still be churning out 80% power or there abouts.

I'd check that inverter spec and make sure its 6Kw. The "5G" I believe is like 5th generation, but there are various models in that 5th generation range as I recall.

Solar Inverters | String Inverters | Energy storage inverters - Solis (ginlong.com)

First rule of unkel: the best solar panel is the cheapest panel per watt. So those 400W panels are better.

Only exception to the rule is if you are very limited for space and a more expensive bigger panel means you can put significantly more power on your roof. This will increase payback time though.

Good man. Most of us make the mistake of starting too small and having to upgrade almost constantly until all our roofs are covered with panels 😂