I'm not sure about Moscow, but six hours in Frankurt is enough time to look around a bit. The subway system connects right to the airport, so its easy to get in and out of the city.
As for what to city, Frankfurt is the financial capital of Germany, so it has quite an interesting downtown from an architectual perspective. With perhaps the Deutsche Bank towers being the most impressive.
Like any European city it also has an impressive collection of churchs -- St. Pauls and St. Bartholeums benig the main two, IIRC. The Romer area is also quite impressive.
My girlfriend also thought the shopping was great, but I can't recall the name of the main shopping street for the life of me right now. Maybe someone else can help me out.
You won't have time to see it all obviously, so stick to what you're most interested in.
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