Mulhouse or Metz

Metz is a much more attractive town than Mulhouse with a better-preserved old city (due to the German occupation from 1871 to 1918 it didn't get "Hausmannised" like most French cities) and more significant architectural sites. The main reason for tourists to visit Mulhouse is for the (separate) French national car and railway museums, both of which are very strongly recommended if you are interested in the subjects but not worth staying in Mulhouse if you aren't.

Another larger town in the same area that you might be interested in is Nancy, which has a well-preserved early modern old town, and was a centre of art nouveau in the late-nineteenth/early-twentieth century, with many surviving buildings in the style and some good museum collections.

Will you have a car or are you using Public Transport? The route du vin is a lot easier by car.
Nancy and Metz are a long way from Colmar. By train you would have to go via Strasbourg, and I think they are too far for a day trip by either car or train.
Mulhouse is nice, and easy to get to by train from Colmar.
If you have a car I would recommend Freiburg in Breisgau on the German side of the Rhine. A lot nicer than Mulhouse. You need a car as there is no good public transport from Colmar.
Also Basel. This is south of Colmar, past Mulhouse, and easy to get to by train or car. Again more to see than Mulhouse, and closer than Nancy or Metz.

Hi,

I've been to Metz a few times, don't know Mulhouse. Given the two, see Metz since it's the first time for you. Historically, it's important. The city was fought over in 1870, 1918, 1944 with the Americans on the last two dates. If you are into the history, have the time in Metz, and are motorised, see also Gravelotte or Thionville as a day trip from Metz. The public transport is only by bus, which I took to Gravelotte from Metz. If you're NOT into the history, then I suggest Nancy instead.