We will be there for few days only to see christmas markets with decorations. I love taking stunning pictures.will be with our 5 years child.

We will only stay in one of this places (sttasdbourg/colmar) and visit for one day the other one. Last day before flight will visit Basel
I know colmar is more crowded which might be a reason to stay since could visit markeys early or late but I have bern told strassboirg is more decorated and beautiful during Christmad.
Also little amusement park like small ferry wheel etc for the child would be a big bonus.
What do you suggest

https://www.waymarking.com/waymarks/..._Alsace_France

Strasbourg, but make sure your hotel has a car park

Both cities have amazing, old carousels. In Strasbourg, it is right near the cathedral, can’t miss it.
The one in Colmar is wood, a treasure, and very large. It is glass enclosed to protect it, but that makes it nice for riders on cold days. There is a crepe stand nearby. A five year old might like the clock in the cathedral, even if they don’t understand time yet. You can google pictures of both carousels.

We're ending up staying in Strasbourg and will also visit Basel (before Strasbourg, for us), where on advice from others here we will be having tea at the Tre Rois Hotel--if you're interested, book ASAP--the Dec 3rd 2 p.m. tea was already completely. booked!

I will always prefer Strasbourg for the Christmas markets. Here is an old report of mine about the various Christmas markets.

Christkindelsmärik of Strasbourg (Christmas Market) - by K2 | Any Port in a Storm (proboards.com)