I have never stayed at either but have stayed at Bucuti and that is near the Manchebo and it is a beautiful beach. The Amsterdam Manor does look like a lovely place but I have to agree with Hatteras about the concern over sewer smell. I was at the Blue Residences down the way and found a sewer smell and I was told that it can happen at Amsterdam Manor as it is near the plant. Since sewer smell is hot or miss, I might be more inclined to go to Manchebo.
The smell just after the Blue Residences is awful IMO, it was there all week when I was on the island a couple of weeks ago. That said, I could not smell it at all when near the AM. I would not think about staying at Blue because of the odors.
We've been staying at Manchebo annually since 2004. The only other place we've stayed one time there was the Hyatt for a few days last year. We are definitely low rise people. We love Manchebo because of the location. In my opinion the BEST beach in Aruba! The rooms are basic but have been renovated and are nice. They do have a microwave and fridge so you can buy food for breakfast and lunch. You don't need a full kitchen. There are only around 70 rooms and there's never a problem getting a hut on the beach. They are also spaced far enough apart for privacy. There are restaurants and a casino right across the street. The Manchebo also has two nice restaurants.