To see the polar light, three things are needed - there polar light itself, no obstacles between you and the polar lights (i.e. clouds, but also mountains if the polar light is low) and not too much light pollution.

Let me first present the case for Tromsø:

• In the right time of the year, Tromsø is right below or right south of the typical polar light belt.
• If signing up with a polar light sightseeing tour group, Tromsø is the perfect place. Those tour groups checks all the forecast information there is out there, they know when it's most likely to be a polar light, and there is almost always a place within one or two hours drive from Tromsø where there is no clouds.
• Tromsø also has an airport, with quite frequent flights to the Oslo airport, and quite some international flights as well - so it's probably one of the most available places in the polar light zone.
• It's also a relatively warm place compared to other places on the same latitude. Almost anywhere else with the same closeness to Aurura Borealis, and you'll be likely to freeze a lot. The Northern Norwegian coastline is unique by being at such a high latitude but still having relatively comfortable temperatures.

Now to the cons:

• In Tromsø itself is quite often overcast.
• There is a lot of light pollution in Tromsø, and it's hard to avoid it since the town is located on an island. Sometimes the polar light is bright and vivid, and clearly visible even from the town center - but it for sure gives a much better experience to observe it from a dark mountain top without direct view towards the Tromsø town. And, even if the polar light is visible if looking at it, it's a chance that one even won't notice it while walking along the lit streets.
• One has to be outdoor to notice it's a polar light at all - and when staying in Tromsø in the winter time as a tourist, it's quite likely that one will spend most of the time indoors.

I was living on the island, but quite far away from the city center, we had no neighbours and no lights except for our own light, and we also had the toilet outdoors, meaning we frequently would go out and observe if there was a polar light or not. I estimated that by average the probability was around 50% of seeing the polar light when staying for a week in my place.

Now that area is a residential area, I sold the place, it has been torn down and new ugly buildings dominate the area ... and lots of artificial light.