Aruba and St. John are very different types of islands. I think St. John is prettier, but I prefer Aruba. It's really a tough call to choose one over the other. Aruba is a bit more predictable as far as the weather is concerned. It's a bit arid, so you don't normally have to worry about getting rained out ... and remember, less water also means fewer bugs. If I had to choose one over the other, I'd lean toward Aruba.

One MAJOR thing to consider ... you say you were already booked for St. John. A passport is not required for St. John, so you may not have one. (?) A passport IS required for Aruba. I thought I'd throw that out there just in case.