The Micra's definitely loud on the highway. It's hard to make small cars not loud, because there's not a lot of weight available for insulation and you're generally close to the road.
I drive 50km every day to work, all highway driving. But you get used to it, and I listen to podcasts/music anyway. So you don't notice.
The appeal of the Micra over a Spark - to me, anyway, everyone has their reasons - isn't aesthetic or comfort driven, if you'll allow the pun. It's price and reliability.
Where I come from (the Caribbean), Nissan, along with Toyota and Honda, has a reputation bulletproof reliability. I bought a Micra S because the price was absolutely unbelievable for a car with so few features there's even less chance something will break or go wrong. Plus the gas mileage I'm getting is probably only bested by a hybrid or electric.
If I were into comfort, quiet and features, my opinion is that the choice is less clear cut; I think at the Micra SV's pricepoint, there are some other good options.
Enjoy your Spark!