You choose based on first....

How many units do you expect to build....

What countries you want to sell in and connect to the PSTN's there....

Then if you make 1000's and 1000's you consider circuits designed with discrete components to make cost be as low as possible...but you also face the fees and time to do agency testing suitable for all countries where you will sell.

If you make just a few select a ready made DAA such as Cermatek that has already an Agency approval in the countries you desire to market in. In some cases the ready made DAA comes with the approval sticker that you can paste right on the back of your device.

Note that not all ready made DAA's are universal the world over and not all carry approvals for every country...so you select based upon approvals that meet your needs, cost of the DAA, and availability.

Normally the issues like package size are the last things to consider becasue the other issues are way more important. After these higher level issues are resolved you end up designing your product around the DAA you have selected.

Michael Karas