Ubud and Nusa Dua balance each other out nicely IMO and doesn't involve an unreasonable amount of travel. I can't speak to all the Marriotts in Ubud, but as far as locations and general standards of service for what you've expressed, Westin Ubud does seems like your best bet. I have mixed feelings about Sthala (would return, but dislike the location and found the breakfast hall to be dark and disorganized). I've not stayed at Element Ubud (thread here) but it's Westin's limited service brand, so it just seem like you could do better for half of your precious 6 nights in Bali. To be clear, I love the grounds, pool, Platinum cocktail hour, F&B team at Westin Ubud, just warning others that the side of the building your room is on plays a huge role in how much you'll enjoy your stay.