Great question! This will also come down to personal preference since both shirts are stylish and comfortable enough to wear all day long.
ENZO: The Enzo is our lightest, most breathable performance tee. It has a little bit of stretch to it, and is moisture wicking so if you are going to work up a sweat it will keep you comfortable. The Enzo is slim fitting so depending on how you like to wear your shirts you might want to go up one size.
AVIATION: The Aviation is our tri-blend tee, with more of a your traditional /casual feel to it as opposed to the high performance Poly blend we use for the Enzo. While still made with performance elements like 4-way stretch, we built the Aviation tee for all-day style and comfort, to be worn in the gym, around the house or for a night out with friends. It has a split hem and a slightly longer cut. Fit is true size, so your normal shirt size of choice should be appropriate here.