I have stayed at both properties on Maui and IMHO the Starwood (Westin) studio is much nicer than the Marriott studio although I have not stayed in the new purpose built Marriott Lahaina Villas so I can't speak to their studios. The Westin has a 2 burner cooktop, a separate tub and shower and a washer and dryer none of which the Marriott have. The original Westin buildings do not have studio lanais with the exception of a few units The new North Westin property has lanais for the studios. The Marriott also has lanais for the studios. Location is another thing altogether. I prefer the location of the Marriott. I can easily walk to several resorts and Whalers Village along the paved beach path however many people prefer the more laid back atmosphere of the Westin property. I think the Westin has a nicer beach area but it is a very short walk from the Marriott to a nicer beach area. I also prefer the Marriott pool to the Westin's but I don't know how much the new Marriott buildings will impact their main pool.