I have stayed at both JW and Westin Lake LV. They are rather different properties. JW is an off strip casino property while Westin is a true resort without a casino. JW rooms are larger and suite upgrade chances are good. But it feels like a typical Vegas hotel and casino is somewhat down market. Westin is set by a lake with beautiful view and true resort feel. OTOH, rooms are smaller and upgrade usually only to alcove junior suite. Westin could have been a desert resort in Arizona or New Mexico.
Personally, I much prefer Westin but I understand some may prefer JW. It is a matter of personal preference.