Of the 2 that you mention, I'd be inclined to go witht he Kobalt for the higher duty cycle as well as the fact that C-H is generally a "cheap" tool.
The pump on that Kobalt looks like the same unit BelAire's use; which is a pretty well-respected brand and may be the actual MFG of that entire unit (Kobalt is Lowe's house brand).
I'll have to agree with Pat that a higher PSI unit is well worth the extra $$'s if it fits the buget and future (or present) needs; You can find 2-cyl, 2-stage units on the 60-gal tank footprint for a couple hundred more $$s.
Check the ratings carefully while you shop; air compressors have brought out the worst case od scum-bag marketing in the tool world. The reason they are now rated "peak" and "running" HP is due to class-action lawsuits against misleading the consumer. The same is going around with the CFM rating now; the rating may be "displaced" CFM which is derived mathematically based on an impossible 100 percent V-E of the pump, or, "delivered" CFM, which is what the unit's true capabilities are. IF you can confirm delivered CFM; that's the spec you really want to compare, but at the very least be sure you are comparing apples to apples.
Aluminum pumps
AFAIK are all MFG'd by DeVilbiss, and have iron cylinder liners. They wear the same as the iron pumps, but tend to cool better.