They are different animals, with differing criteria and results.
Fraud alert criteria are clearly set forth under FCRA 605A, while freezes are not explicitly defined under the FCRA, and are based on CRA policy of upon state laws or regulations.
Has there been any suspected fraud or identity theft upon which you are basing your request, or are you simply seeking to freeze your report?
If suspected fraud or identity theft, do you have an actual situation upon which you can file a police report? The distinction is that an initial fraud alert under the FCRA requires only an assertion of expected identity theft, and lasts for only 90 days, while obtaining an extended fraud alert requires the additional filing of a police report with the CRA.
What is your specific situation, and what are you attempting to block?
I'm not sure where you would have read the suggestion to freeze secondary consumer reporting agencies but its definitely a minority opinion around here, and as far as I can tell it only leads to additional problems. Lenders want to know about you if they are going to extend unsecured credit to you. So freezing these sources of additional information will only result in denials.