I have had some angels reach 6 inches in a matter of months and some that never did. It depends partially on gentics. Either way, unless they are stunted, I don't think 2 adult sized angels would be happy in a 20 high. They are not the most active fish I have ever raised, however they make full use of my 150.
You are right in some ways with your comparison of the oscar...they eat a lot more and are much messier. However, I agree with Rbishop...the fins are important to consider in the overall health of an angelfish. If the fins are constantly in contact with substrate for example they can easily develop fin rot. So, it's not really just a "biomass" issue.