JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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If I understand you correctly, you would like to know if it is better to use the source code instead of the iFrame script to embed the form into your site.
Using the iFrame script will eliminate any conflicts the form may have with the site. You will also be able to update the form your jotform account.
When using the source code, you have a change of getting conflicts between variables of the form and your site. This will make your site not work as you like. If any changes are made in the form from your jotform account, you will also need to change it manually in your site. It will not get reflected automatically.