Trying to decide which city to cut from my trip planning

I’m planning 2 weeks in December (I know it’s early, I like to plan far in advance). It’s our first time out of the country together and her first time entirely. We will definitely be spending a week in Paris, but I’m struggling to decide whether to cut Reykjavik or London out. I would really like to see the northern lights and snow (from southeastern US, never seen snow that stuck to the ground), but I also really want to take the train from London into Paris. The trip is a surprise for my partner and I ideally would like to propose on this trip, which is why I’m so hung up on the northern lights, though I know that’s kind of a gamble.