Never stayed at the other but I would 100% stay at Cosmo .
They are two different worlds. Cosmopolitan is vibrant, modern, alive 24/7. Westin Lake Las Vegas is remote, quiet, and relaxed. Depends on what you are looking for.
To be clear, are you confusing the Westin Lake Las Vegas Resort & Spa with the Westin Las Vegas Hotel & Spa, which is located just off the strip?
As stated, what are you looking for? Your answer will guide you.
If you are hitting a couple lf shows it seems silly to drive back to Lake Las Vegas at the end of each night.
Cosmo I’d even stay at the Flamingo over anything at Lake Lasvegas.
When it is very hot the "lake" reeks. I don't know if the hotel is close enough to the lake so it might be an issue.
Cosmopolitan fountain view. Beautiful.