I think either would fit your requirements, but the two cities are quite different. Both are interesting and worthwhile, both would have more-than-enough attractions to keep most folks happy for several days. We visited both briefly last fall (late September), without a car.
But Strasbourg is a major city, with world-class monuments (the cathedral is incredible), it definitely feels like a city (though a charming and very historic one). It's a big, busy place, lovely and charming, easy to love.
Colmar, just down the road, is much smaller (feels more like a large town than a big city), and is overflowing with cuteness, which is both good and bad. The "cute" stuff in Colmar is dense, with more "cuteness-per-square-mile" than just about anyplace I can think of. This has made it extremely popular with instagrammers and others who can't get enough of the "Hello Kitty"-level charm. While there's no denying Colmar's appeal (who doesn't like cute?), I found the crowding and the kind of crowds we encountered there kind of depressing (my wife didn't care, she was as intoxicated by the gushy charms as all the others were). I've never see so many selfie-sticks before. I found it nearly impossible to take a picture that didn't have a dozen women (mostly young, often dressed in what I would regard as theatrical outfits) grinning from ear to ear, obviously posing for social media posts. I tired of this pretty quickly. Don't get me wrong: Colmar is certainly pretty, charming, oh-so-cute. I just have so much tolerance for the crowds that attracts (yes, I'm aware of the irony).
We did a day trip from Colmar to Eguisheim, one of the nearby "wine towns". That was much more my style. First, it was almost as over-the-top cute as Colmar, but it felt far more authentic - the town actually smelled like wine (I lived on a winery once, and I know exactly what Chardonnay and Gewurztraminer smells like - the streets were filled with that fragrance - so I liked the place right away). Yes, it was touristy, but it attracted far fewer tourists (thank goodness) and a clearly different demographic.
In Colmar we saw mostly Asian tourists, mostly women, either young-ish (with the selfie sticks and flowing gowns) or older (presumably their moms, aunties or grandmas). All appeared to be completely in bliss. Not a lot of round-eyed devils like me among the tourists. It was like that in Colmar all day for the two days we were there - it's obviously extremely popular with social media and folks visiting from China, Japan, Korea and Thailand (my spouse, who is is Thai, told me that, so I'm not just guessing, and these were the languages we heard).
In Eguisheim, when we got there (mid-morning) there were actually relatively few tourists on the streets - my heart skipped a beat! Late in the morning we saw more tourists, but there was a definite demographic trend: lots of pasty-looking, older white couples, many clutching a familiar blue guidebook. Yes, the Rickniks were there. As the day wore on, it became somewhat more crowded and the crowds diversified a bit (by noon, the first instragrammers in their wedding gowns waving the selfie sticks appeared). We departed mid-afternoon, but the crowds never got anything like the dense packs of people we encountered in Colmar. I liked Eguisheim a lot more than Colmar (my spouse loved them both but grudgingly admitted that Eguisheim seemed more real and less of a Disneyesque experience). If I had a few days to spend there, I'd seriously consider a full day and a night in Strasbourg, another in Colmar, but don't miss a day-trip to Eguisheim.
I'm sure others have their preferences which will differ from mine. Can't help you with any German side-trips. Bonne chance.
