The pump pillow and the gamechanger are very different products. If you can spring for both you will be happiest. The pump is great for adding stability to your position by filling voids like spaces between knees and elbows. The gamechanger is VERY popular because it makes a solid base to rest the rifle on obstacles, especially railings, logs and so forth.
At the New England PRS match a few of the top shooters were overheard telling new shooters to buy the gamechanger first if they could only afford one bag. Also, the older version is better, but no longer available.