Well Carol has said that I am the expert on La Terrasse. Not too sure about that. I will try to give you more information and if you have other questions, ask away.
La Terrasse was built in 2003 and began operation in 2004. So it is roughly 20 years old . La Plage was built in the mid 1980s so it is older. La Plage has only timeshare units and there are 132 units in the building. La Terrasse is a mix of privately owned condos and timeshare units and there are only 76 units. I am a private condo owner in La Terrasse. The interiors of the private owned units very significantly.
Both buildings have been renovated more than once. Although units have similar decor, they may differ slightly inside.
La Plage has beachfront access and there are many palm trees around the pool. That is a nice atmosphere but it is significantly busier around the pool since there are more guests. The palm trees may also affect the ocean views from the room.
La Terrasse, is a quieter building around the pool area, and there is a nice view of Maho Beach. Of course, if you’d like to watch planes, you will be watching them very close. . There is a pool bar which operates mid afternoon until roughly 7 PM. Of course you can walk to the beach by going through La Plage or walking across the road from Maho market and towards Driftwood Boat Bar.
The penthouse units at La Terrasse have very large rental spaces and their balconies are very large. They have their own umbrellas and seating areas. The penthouse unit you would be renting overlooks the pool bar and pool. I have attached a picture. The penthouse large terrace would be a big plus as it would be your own private space with an amazing ocean view. As far as the interior, I would email Nina at Royal Islander and ask if she could send you some interior unit photos of the penthouse.
Not sure if that helps much. Both are good choices for what they offer..