I looked at possibly getting the xB over the Flex (drove the xB first), but in all honesty there is no comparison; they are alike only in shape.
The XB is a great smaller car with a decent amount of cargo space, but the Flex is much closer to a full-size SUV (without the height). The flex is much larger, with seating capacity for 7 vs. 5. For a more direct comparison, the amount of cargo space in the xB with the back seats up is about the same as the amount of cargo space in the Flex with all three rows up. The Flex also has a 3.5L V6 vs. the xB's 2.4L I4 (and the Flex needs the bigger engine purely on size).
Interior finish on the Flex is also much different than the xB. The xB feels very economical, while the Flex, while not luxury quality (at the base model), isn't made up of so much drab plastic (I know, amazing for an American car). This same step up carries over to ride quality and ride noise. The xB rides very well, but it is still a small car on a small car frame. The Flex, on the other hand, uses it's weight very well to handle bumps with grace, and the cabin is well isolated from road and wind noise (the xB is so-so on road noise).
Overall, they are just very different cars targeted at very different age groups and uses. The price split between the two is around $10k, and I promise you, the Flex is $10k more worth of car.