This is the free Macrium version and it takes full images of the 20GBs from the SSD to the HDD. I do not like differentials because if you lose one in the middle, you lose the whole chain. Even when I used to image with Ghost, I always took full images. I do keep all my data in a different data partition. That partition I image too, but on a different schedule.
Windows7 has an imaging facility, but I do not like it because a), I have seen a lot of people that had trouble with it and b) you cannot mount the images to e.g. copy just one file without jumping thru 7 hoops.
I trust Macrium a lot more and I have made recoveries on Windows7, Vista and XP at least a dozen times without incident.