Note that these used to be a single "common" share that has "split" (actually a "special dividend" but effectively a split).
If you owned one share of Google before the split, you had one share giving you X worth of equity in the company and 1 vote. After the split you have two shares giving you the same X worth of equity and 1 vote. In other words, zero change. Buy or sell either depending on how much you value the vote and how much you think others will pay (or not) for that vote in the future.
As Google issues new shares, it'll likely issue more of the new non-voting shares meaning dilution of equity but not dilution of voting power. For most of us, our few votes count for nothing so evaluate this as you will.
Google's founders believe they can do a better job running the company long-term when there are fewer pressures from outside holders who may have only short-term interests in mind. If you disagree, or if you are only interested in the short-term, you probably shouldn't be an owner of Google.
As always, evaluate the facts for yourself, your situation, and your beliefs.