Bryan,
One way to answer your questions is to EM simulate the various inverted F antenna (IFA) artworks and compare the s-parameters. Sonnet SW distributes "Sonnet Lite" free of charge and it's easy to learn and use. I also recommend MWO Axiem and Agilent ADS Momentum -- all will easily simulate the IFA's.
Also, have you considered using a COTS antenna? They will be smaller if a higher dielectric substrate is used. However, I don't know how cost sensitive your project is.
Regards,
Eric Hooker