Multiplication vs exponentiation in pairing based cryptography
The problem is that you are comparing apples and oranges. A group has a single operation associated to it. Depending on your notational conventions you either need to write (1) $e((a+b)P,Q)$ and (2) $e(aP+bP,Q)$ if you write $G_1$ additively or (1) $e(P^{a+b},Q)$ and (2) $e(P^a \cdot P^b,Q)$ if you write $G_1$ multiplicatively. The cost of computing $(a+b)P$ or $aP+bP$ depends on your approach to scalar multiplication, but typically the former approach will be more efficient.