My husband and I went to Strasbourg for 4 nights and Cologne for 3 nights last year. They were very different experiences. I would watch some videos of the cities in general, not just the markets.
Strasbourg and the surrounding villages are fairy tale settings with adorable half timbered building. They go ALL OUT with the Christmas decor. We were in Colmar on a weekday morning when giant snowflakes were falling. It was like being in a snow globe! I was sooo happy.
That said, Cologne is a modern, bustling city. It was HEAVILY bombed in the war, so it doesn’t have that old world half-timbered vibe. BUT the markets were our favorites. They were fabulous with unique theming, GREAT food (much greater variety than in France), great entertainment and more variety of items being sold. We LOVED the markets in Cologne they were our favorites of over 20 markets), but the ambiance (citywide) in Strasbourg is better.
We had been to both areas previously, so our trip was focused solely on markets, not other tourist sites.