It appears that the pricing is uniform regardless of the type of card used:

Transparent Pricing

• 2.75% per swipe for Visa, MasterCard, Discover and AmEx.
• Free app, free Square Card Reader and free shipping.
• No merchant account, monthly fees or set-up costs.
• Manually-entered cards cost 3.5% + 15¢ per transaction.

As for cash? Just take the cash!

Coyote