The only real difference will be in rolling diameter. Basically the Qashqai tyres have a rolling diameter of 2167mm, the primera tyres 2032mm. What this means in practice is that your sppedo will OVER-read. In other words, with the existing tyres if currently reading 60 mph it will read about 64 mph. If you drive to the same on the speedo, you'll actually be going a bit slower, so will probably end up getting slightly better mpg.
However, you really need to check if the 215/50 tyre size is 'officially' approved for the Qashqai (have a look in the handbook, or on door pillar for the tyre size/pressure label. If it is NOT listed, then technically your insurance could get bolshy if you had an accident, as you weren't running with the right tyre size on.
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