For snowboarders Ruka is nowadays the number 1 resort in Finland... Levi and Ruka have both been in turns voted as the number 1 ski resort in Finland. There's no major differences between them. Both have nowadays a built up Alpine village type centre where the majority of the services are concentrated at with vast cabin areas surrounding the actual "core" of the resort.
Crystal will operate flights from London to Kuusamo airport Dec 13 - Feb 19 this winter.
Usually around Christmas and New Year Ruka has more ski runs open than Levi where they open most of the slopes only for the peak season in February. Ruka is traditionally the first ski resort in Europe (outside glaciers) to open the ski season which takes place usually in mid October (the set goal this year is 14 October, weather permitting).
Ski lifts in Ruka probably better suited to snowboarders: 5 chair lifts, 7 T-bar lifts, 3 ring lifts, 3 rope lifts and 2 carpet/conveyor belt lifts.
In Levi there are 1 chair lift, 2 gondola lifts, 14 T-bar lifts, 5 ring lifts and 4 rope lifts.
You can compare the slopes yourself: http://ski.ruka.fi/eng/ and http://www.levi.fi/en/pistes-and-tracks/downhill/facts-of-levi-lifts-and-pistes.html