Back then when I was doing my research for a RAID card and found some extra Percs to sell, I also "ran" into this situation.
Basically when I was handling Perc5's, the difference between the Adapter and Integrated was that the adapter was the one that came with a PCI bracket as it was meant to be used in a tower that could fit the entire card depth while the integrated came with a backplate instead of a PCI bracket because it was geared more toward server racks and would be attached laying down since it wouldn't be able to support the depth of the card if you were to mount it as usual in a tower.
Performance and specs wise, there's no difference. You just have to make sure to get the right firmware/drivers that is specific to your card (i.e. - adapter or integrated)