Northern Norway or Finland for Northern Lights in March; Ice Hotels?

The northern lights are tricky. For successful viewing attempts, a number of factors need to coincide. You need darkness, clear skies, low ambient light, wide horizons, and most importantly, a solar flair storm, which isn't predictable more than a few days in advanced. Solar storms can occur nightly for weeks on end, not at all for weeks at a time or at any frequency in between. There is no way to predict. This means that if you simply pick one or two nights for viewing months in advanced, you will likely be disappointed. So, I would choose whichever destination interests you more, and take the presence of any aurora borealis as a pleasent side-benefit.

For comparison, I once spent a month in the central Alaska during the winter. I checked the online aurora forecast, and on nights with a high chance of aurora activity, I drove to suitable viewing location, bundled up, looked towards the skies, and waited... and waited... and waited some more. I wasn't successful until the third attempt. In one month, I only saw them three times.

There is a very comprehensive FAQ on northern lights over on the LP forum https://www.lonelyplanet.com/thorntree/forums/europe-scandinavia-the-nordics/topics/faq-seeing-the-northern-lights-in-scandinavia-iceland-and-the-nordics

Thank you!
This is EXCELLENT!