My wife prefers the shopping in Aruba to both St Thomas ans St Martin. Finds to shops a bit nicer, prices competitive and overall experience a bit nicer. There are three major shopping "malls" within walking distance and dozens of shops on the main streets in between and behind the main street.
Your best bet is to know what you are buying and what you would pay for it in the states.
For jewelry, always ask for a better price, the same for electronics.