Your credit score will decrease (somewhat) because of the hard inquiry from opening the account. This effect will diminish after a few months. Your credit score may increase because your credit utilization ratio goes down as you open a new line of credit. Your credit score may decrease/increase depending on whether you have too many/too few open accounts per their scoring format.

In the end it doesn't really matter unless you're planning on applying for a loan in the next few months. If you are, don't open a bunch of cards for the miles. If you're not, why worry about your score?