Saracen's bay is no longer the unspoilt paradise it is often described but it's definitely not the circus of Koh Phi Phi for example. The is no stretch of undeveloped beach along the long bay. The hostel are spaced apart and very few encroach on the beach. More of a resort with no Khmer families seeming to live there. There are hotels with infinity pools and $15+ mains. Still a few dorms and rooms below $20 but few cheap eats, it's backpacking days may be numbered.
Mbei Bay is much more rough and ready, a decent amount of construction and the main beach could do with a litter pick. Hotels all backpacker focused and cheap eats a plenty. Nice village feel and Khmer families living there.
Koh Rong Sanloem can be reached by boat from Sihanoukville in around an hour, don't always leave on time but generally punctual for island time. $10 each way, most will only want to sell you a $20 open return. Currently many tour operators offer daily trips to the island. There are also multiple fast ferry options available. EcoSea Dive offers overnight trips to their bungalows on the northern part of the island and The Dive Shop  runs a daily ferry from Sihanoukville to the sunset beach located on the western side of the island.
You have two choices in your style of boat to the island. You can make it into a day trip and go on either of the cruise boats Party Boat or Sun Tours. These take a whole day to get there but its an enjoyable ride, including lunch, snorkeling trip and drinks at the bar onboard.
That is fun but if you don't care about that and just want to get to the island as quickly as possible then its best to go on the fast catamaran which takes only 45 minutes Speed Ferry Cambodia