I'm no expert....but range of adjustability is important. Adjustable distance between arm pads (which allows you to bring your arms in for more "aero" or move them out for more comfort/stability, adjustable length of the bar extension--you might have long forearms, and perhaps adjustable height of armrests. The bar should fit your handle bar diameter--which usually isn't a problem.

Personally, I've always found the basic, less expensive bars just fine...but I wasn't racing the clock and aero wasn't my intention so much as comfort.

** There is a model aero bar that has spring loaded arms which flip up when you're not using them to allow your hands access to the regular bar sleeve area around the stem. Personally, I'd avoid these. The arms take more effort to get down on and, worse, they rattle when in the up position. Drove me nuts!