Skipping sums

Algebra Level 3

If $a + a^2 + a^3 + a^4 + a^5 + \cdots$ is a positive number, then which of the following is larger,

$a+a^3+a^5+a^7+\cdots\quad \text{or}\quad a^2+a^4+a^6+a^8+\cdots\, ?$