One counter argument might be:
if you do another night this year you get Concierge for the rest of this year (!) and 2013. And no nights credit carried forward.
On the other hand if you leave it at 74 for this year your concierge remains valid until Feb 2013 but you drop to gold. However you carry forward 39 nights credit (=74-35) so will only need a further 36 nights in 2013 to make Concierge for the rest of 2013 AND 2014!! That's actually a net cost of an extra 35 nights since you won't have done the one this year.
depending how quickly you can do 36 nights in the new year, and whether you would be likely to re-qualify as Concierge in 2013 anyway, the second option may be better!! especially if you could do those 36 before end of Feb.