Help with Hop Choices

I'm using BeerSmith to help me learn how to formulate various recipes. I'm a little confused about hop selections in some recipes though. Some hops are listed as bittering and some are listed as aroma, and some are listed as both.

Are the hops that are listed as aroma hops poor choices for bittering hops? If so, why? What determines whether a hop makes a good bittering hop or aromatic hop?

A high alpha acid percentage (AA%) is primarily what gets them classified as "bittering" hops. Typically (but not always), the double-digit AA% hops will be listed as bittering, sometimes as bittering and/or flavor and/or aroma, depending on (naturally) their flavor and aroma. Some of the high AA hops also have good flavor and/or aroma characteristics (Centennial, for instance), some don't (such as, IMO, Chinook).

The high AA hops are typically used more often for bittering also because if it's bittering you're after, it's more efficient (cheaper) to use high AA hops near the beginning of the boil than to use the same hops for flavor or aroma (since you get less and less bittering from hops used nearer the end of the boil).

Hops with subjectively better flavor and aroma characteristics are more often used for flavor and aroma, regardless of AA... and the lower AA hops are used less often for bittering because you need to use more of them to achieve the same bitterness.

Not to say that hops boiled for 60 minutes (bittering) won't lend any flavor at all; some of them will, some of them won't, but it won't be a lot.

So, if the recipe calls for 1/2oz Cascade @ 60 min and 1/2oz Cascade at 5 min, this is for low bitterness and low aroma (assuming ~8% AA?)? If I want to substitute Mt. Hood for the 5 minute addition, even though its AA is less than half that of Cascade, should I still use 1/2oz as the recipe calls for, or up this to compensate?