They both have their pros and cons. To make it short, GT is the more well polished game. But what GT lacks, PCars makes up for. The wide variety of cars and tracks in PCars craps all over GT. But theres just something about the driving physics in PCars (and bugs, hence PBugs) that leaves a bad taste in my mouth.
If I had to choose one to play forever I would choose GT. But give PCars a shot its fun. But if you do, dont start calling GT an arcade racer and leave SNAIL completely (
cough cough @aerolite @Mike Lobban @GTLehman @SGETI @Alex ONeill @Gr8_Lakes)