You probably won't notice a difference between either. If you look at the last few years of NAF (Norway) winter tire tests, the same three tires seem to trade places in the top three or four postions.
• Michelin X-Ice Xi2
• Continental ContiVikingContact5 aka ExtremeWinterContact
• Nokian Hakkapeliitta R
The Bridegstone Blizzak WS70 was finally included in the 2011 test and it came in 5th, just behind the ExtremeWinterContact.
TireRack.com tested the Blizzak WS70, X-Ice Xi2 and ExtremeWinterContact and they all have very similar performance.
Frankly, I'd go with what ever you find the best deal on. Every test shows these tires are so similar that you'd need special test equipment to even tell the difference.