Rose (Defense '10/11 - best of the last two seasons/due to this season being a sample size e.g. Off-screen - 0.36 PPP):
Overall - 0.77 PPP allowed
Isolation - 0.61 PPP allowed
P&R Ball Handler - 0.73 PPP allowed
Off-screen - 0.80 PPP allowed
Spot up - 0.92 PPP allowed
Westbrook (Defense '11/12 - best of the last two seasons):
Overall - 0.88 PPP allowed
Isolation - 0.83 PPP allowed
P&R Ball Handler - 0.85 PPP allowed
Off-screen - 0.98 PPP allowed
Spot up - 0.99 PPP allowed
"Rose has allowed just 0.77 points per possession overall on defense this season, an elite mark for any defender, regardless of position. Chris Paul (0.86 points per possession allowed), Rajon Rondo (0.83 PPP allowed), and Russell Westbrook (0.92 PPP allowed) - all excellent defenders - have been trumped statistically this year, and by no slim margin. Rose has each of those players handily beat, and boasts a shockingly comprehensive defensive profile. Rose is particularly effective in defending isolation sequences, where he allows just 0.61 points per possession."
Rose had more blocks than Lebron last season + 1st overall blocks for a PG (21 more than 2nd place) + 2nd overall perimeter blocks for a guard (3rd overall blocks).
Said statistics are certainly not the be-all and end-all to thee argument, as it doesn't weigh in eye-tests; team-defense/help-defense influence; opponent-matchups; weight of offensive-load; defensive-intangibles - but it gives you a general perspective of where they both sit, in terms of defensive ability/efficiency.
Bottom line - Westbrook isn't a much better defender and it's certainly not crazy to think Rose is the better defender in any way, shape or form.