difference between Portal access resource and webtop links in full webtop

One very specific difference is that Webtop links point directly to an external URL and when clicked the browser will connect directly to that site without passing through the BIG-IP at all (unless of course you have a Network Access tunnel without split tunneling). Whereas a Portal Access goes through the BIG-IP, and as Michael says the BIG-IP will rewrite content for that application.