Too hard to tell but it's definitely competitive ...
FIFA 18 is a new game releasing on a system where the console manufacturer is weaker than ever in the region which the game performs best in and Skyrim is an old game is releasing in the strongest performing region of the console manufacturer which is the game's best performing region ...
I think FIFA 18 might top Skyrim for the Switch all things considered and portability is not so much of an argument in favour of Skyrim as it is for FIFA 18 since the feature is inclusive to both ... (Skyrim is also going to undergo a second rerelease which will water down it's sales even more.)
To be clear, Nintendo has had weaker periods in Europe. The NES, SNES, and N64 underperformed there.
The bigger problem with Fifa is that the Switch version is universally recognized as objectively inferior to other major versions. That it looks a bit better than the PS360 version as EA declares doesn't change that it is missing the big new feature for the game. As a result, I've seen a lot of rolling of eyes with regards to Fifa. Skyrim, on the other hand, has had a pretty good reception. It looks like a much higher effort port, is feature complete, and seems to be at least derived fromthe Special Edition release. I've seen much more positivity with Skyrim. And the title could very well experience some synergy with Zelda as a lot of people who loved that game and loved having it on the go may be looking for a similar experience.
Honestly, a better comparison would be NBA 2k18 and Skyrim, two feature complete and good effort ports.