I have been diving in Aruba twice. The first time I used Red Sail, which I had an ok time with. I think that they have changed a bit, there is a lot of information in the boards about them. The last time I went diving with Diveversity and I won't dive with anyone else.
They are located very close to the Marriott, just behind Momba beach. The max they will take out is 10 with two instructors. They are a smaller shop that offers great service. They also have new gear (it is all less than a year old). They really do go out of their way to help folks out.
The other shop that I have heard good things about is JADS. The only draw back to them is they are on the south part of the island, so from the Marriott, it is about a 40 min car ride.
HTH