2.8 Dual core means 5.6Ghz in total while the 2.2Ghz quad core means 8.8Ghz in total. Just FYI.
Yes there is a noticeable difference between the speed of both. Everything loads faster on the 2.2Ghz quad core as there are four processors making it possible to work on the heaviest process like video editing easily. You can play the most advanced games without any kind of lag situation in any case. From my bookmarks here is an excellent review from the owner of 2.2Ghz 15' macbook pro to help you out on this decision:
www. budurl.com/NewestMBP15Review