If it's a standard html only/javascript site. Use HTML mode first for recording and script baselining. HTML mode parses the html response to the call and "looks" for other resource aspects to also download dynamically. Then anything javascript based during recording is added as "extra resources" or separate calls in the script depending on how you setup the recording options. URL mode records and sets the script to call EACH resource found statically and no runtime parsing is involved. If you are new to PT in general HTML mode is usualy the cleanest/easiest to use first and maintain later. If you use URL mode early on and the developers change anything about the call/response set then you have to modify your script each time accordingly adding or removing resource calls. In HTML mode, for the most part if they don't change any form POST/GET name pairs they can change content without you rerecording.
