Is a penny with a mintmark more valuable?

I'm afraid the answer is "Yes, no, and maybe." It depends on the year of issue and the condition of the coin.

Since the 1940s the Denver Mint has turned out about as many cents as the Philadelphia Mint so D-mint cents from the last 70 years or so aren't generally scarce. You have to go back to the early 1930s and before to find some that are worth any reasonable amount.

As for San Francisco, at one time that Mint was active almost on a par with Denver and Philadelphia, but some years it in fact did mint fewer coins than the other 2 facilities. Also there's a bit of an "air" about S-mint cents because they haven't been struck for circulation for decades and are thus comparatively unusual in most peoples' experience.

Please see the Related Link for a site listing current market values.