Look at the price. If you are on a budget, price will be the largest factor. Section One of Rosetta Stone costs more than $200, while an introductory language course from Pimsleur costs well under $100.
Compare the programs. Most Pimsleur content runs off of a standard CD while you follow along in a book. It is essentially like a language teacher that doesn't answer questions. Rosetta Stone runs off of the computer and is flash-card based, showing you pictures and matching words. You can repeat any card at any time. If you find it easier to learn via pictures, you may want to go with the Rosetta Stone option.
Check out the audio features on the programs. Reading and hearing a foreign language word is one thing, but actually pronouncing it is another. Pimsleur has no audio support for pronunciation. Rosetta Stone provides a microphone packaged with the software, and the program informs you of how accurate you are when you say the word or phrase. Often, it does not allow you to progress to the next flash card until you say the current phrase correctly. This feature helps ensure you pronounce everything accurately.
Check for travel support. Both Rosetta Stone and Pimsleur have audio files you can upload to your MP3 player and play while on-the-go. This mobility helps you stay on top of your language skills.
Look for online support. Rosetta Stone has an online community (included with the price of your software purchase) that allows you to communicate with actual online language professors and participate in online classes and even ask questions. Pimsleur does not have this option.