Been finishing slabs for close to 15 years now and I remember standing in the supply house staring at a wall of trowels with no clue which one to grab. So here's what I wish someone had told me back then. First thing, your second trowel shouldn't be a copy of your first. If you've been running the same one for years, get something that does a diferent job. A smaller blade, like a 12 inch, is great for tight corners and around penetrations. A bigger one covers open floor faster but it's heavier and harder on your wrist if you're not used to it. Also think about the material. Carbon steel is the standard workhorse, stainless resists rust if you leave it in a wet truck box, and there are plastic or flex options for late passes when you don't want to burn the surface. The real trick is breaking it in before it touches a slab. New trowels come with sharp edges that will drag and score fresh concrete. Take a file or rub two bricks across the leading edge untill it's slightly rounded. Ten minutes of prep saves you a ruined finish. And keep your old one, even beat up. A worn blade has its place on later passes when you want a softer touch.
Last thing, don't cheap out on the handle. You'll be gripping it for hours and a handle that fits your hand matters more than most people think. Hold a few before you buy if the store will let you.