If anyone can explain how to prove if one economy is stronger than another in less than 1000 words on this forum I would be very surprised !
You only need two words actually: "Selling Stuff"
That's a fallacy and a common misconception. This has also been the official German dogma of a strong economy since the 50ies, i.e. having a positive balance of payment.
Suffices to say that, for instance, the UK hardly sells anything tangible, yet attracts much more capital than Germany. Likewise, the US which is chronically racking up huge deficits in balance of payments, attracts more FDI than any other country. There is a good reason for that: those two economies are much more dynamic and offer better return on investment than the rigid German one.
Given that you are a notorious Germanophile, no wonder you state such a view.
Which one is stronger?
The question doesn't make sense. It is like asking which is the happiest of the two populations.
As the proverbial "you can drown in a river that is a foot deep on average" goes, GDP per capita, average this and that are mere statistical tools and mean nothing on a individual basis.
Can you actually describe how a "a solid 4% 2nd Quarter growth" feels like for the average Joe? of course u cant. GDP and the like are mere tools for politicians to keep fooling their subjects that they are actually doing something.
You need to look at the question from an individual perspective. Take a Pole and a Ruski at random and see who is wealthier, according to their own conception of wealth, and who is the more successful in his own pursuit of happiness. Whatever conclusion u'll come to, will only apply to that specific case.