change default payment in ubercart
Probably with hook_form_alter. Change the #weight attribute so credit card lighter than paypal. Set the #default value to credit cards for the radio values.

I haven't used these modules nor have an Ecommerce site live so I cant post example code ...

EDIT

install Devel and implement hook_form_alter for your module. put dpm($form);dpm($form_id); in your form atler implementation ... and goto the page with the form in question. You will then see the PHP array structure for the form on the screen.