Differences that probably don't matter much to food photography:
- hard buttons: this is more useful for people shooting outside in cold weather in so far as it allows you to do basic navigation and settings without taking off thick gloves in the cold
Differences that may impact your main area of work:
- color profiles: there are no pros or cons here, just a difference you need to be aware of. The Credo and IQ are profiled differently following different lines of historical priority/taste. While you can, to some extent, work the file in post to make one more like the other, this is never 100% possible and in any case requires work. Some people prefer Leaf color, others prefer Phase color - only you can decide.
- warranty availability of a 5 year VA warranty for the IQ. This includes a license for C1 Pro, a loaner during any service, a few accessories (including the Hassy V mount lens adapter which can be nice as the Hassy F series 110/2.0 lens is nice for shallow DOF food shots) and a free swap of mounts/platforms.
- focus mask: this is fairly useful for determining focus, but it's VERY useful to determining Scheimpflug movements.
- auto horizon/perspective correction: if you want to PC your shot the IQ will allow auto correction at the time of tethered capture or single/batch correction during post processing. This is because the IQ records, in the metadata, the tilt/rotation of the back at the time of capture while the Credo does not. Less important for plates, but can be useful for bottle shots, box shots, and other shots with tall strong verticals.
- price: for obvious reasons
My suggestion is what it always is. If at all possible (and living in Philly where my colleague Lance Schad lives it's very possible) test both backs in a test shoot, try the feature difference to see if they are useful to your shooting style, and spend some time with the files, and then decide based on what suits your fancy. Other's opinions will only get you so far.