Stayed at both multiple times. This summer we will even mix few days at Sheraton with few days at Marriott.

Advantages of Sheraton:
- Great location in the center of Sopot... but if you don't like the crowds (especially during weekends in summer), this might turn into disadvantage
- In summer months they have 'private' part of the beach in front of the hotel where you can get beach chairs, etc.
- There is very good Club Lounge (if you are Platinum or above)
- There are more Suits than in Marriott, but it might be still difficult to get upgrade during summer months

Advantages of Marriott:
- Location is much more peaceful, no private beach but beach in front of the hotel is not really crowded
- Much better spa area - with multiple pools, jacuzzies, saunas etc.

I would say Marriott is more relaxing, better if you are with children. Sheraton is better for a couple if you want to be closer to the city.

Let me know if you have any additional questions.