I was wondering why they were using a more powerful Delta 2 to launch Opportunity than what was used for Spirit. Here's the answer cut from a Spaceflight Now story.

The Delta 2-Heavy is needed to launch the Opportunity rover since the liftoff comes after the optimum portion of the planetary alignment window between Earth and Mars that occurs every 26 months. The Spirit rover was carried aloft June 10 by the standard Delta 2-7925 configuration launcher.

"It is purely an energy thing," MER project manager Peter Theisinger said. "We are out of the sweet spot, low-energy part of the transfer trajectory. So we need the Delta 2-Heavy to launch the exact same payload mass to Mars in late-June through mid-July."