Played both .. Both are awesome games ...
Battlefield 3 :
Better graphics than MW3
Short Single player campaign ..
Campaign mode starts with a Bang ..
Decent graphics but not upto the mark of battlefield 3.. (BF3 graphics was awesome , no doubt)
Single player campaign is better in MW3 than BF3
More cinematic sequence in campaign mode , which keeps you entertained. ( MW3 wins here surely )
Campaign mode starts somewt slowly but once it picked up its pace , thr s no stopping ...
Its just my opinion .. Overall i liked both.... But my vote goes for MW3 ... bcoz its just me ...