In a few months I would like to A/B test a few elements of my merchant site such as trust logos, color scheme, graphics, etc. I would like brainstorm a little on how to implement simple A/B testing.

My question is what technique is best to determine and keep track of which visitor gets A and which visitor gets B. Best meaning a combination of code simplicity and site speed efficiency.

So far I believe I could use either IPs, SIDs, Cookies, or a custom tracking number. Could it be as simple as even SIDs get A and odd SIDs get B and in checkout_success.php the SID is recorded for later processing?

Also if I were to begin to implement A/B testing for Multi-Page Checkout vs. One Page Checkout is there any foreseeable issue? Initially my thinking it would be as simple as modifying all the checks in all the pages that are similar to the example below by adding my A/B check code.

PHP Code:
if (ONEPAGE_CHECKOUT_ENABLED == 'True'){
      tep_redirect(tep_href_link(FILENAME_CHECKOUT, $_SERVER['QUERY_STRING'], 'SSL'));

  } 
thanks for any advice offered