My first structured bag was a boxy little thing with a top handle that I bought because it looked like something off a magazine page, and within a month the corners had gone soft and the base had started to sag like a tired face. What it taught me is that structure isn't about the shape it comes in, it's about whether it keeps that shape after being sat in, dropped on a cafe floor and shoved under a desk for a full day.
If you've got none in your collection, start with something mid sized with a top handle and a detachable strap, in a shape that's a bit boxy rather than a soft curve. That combination does the most jobs. You can carry it by hand for anything that needs you to look a bit put together, or sling it across your body for a day where you need both hands free. A bag that only does one of those things gets left at home more than it gets used, and there's no point owning a lovely bag that spends its life on a shelf.
Fit matters more with structured bags than people expect, because unlike a slouchy tote, there's no give. Check the handle drop against how you'll actually carry it, under the arm needs a shorter drop than over the shoulder, and if you're between the two you'll be hitching it up all day, which gets old fast. Look at the opening too. Some structured bags have a gap that barely fits a hand in, and you'll spend your life fishing for keys at the bottom like you're playing a fairground game. A base that's slightly wider than the top keeps things stable when you set it down, which matters more than it sounds once you've watched a bag topple sideways on a table and spill everything out.
On colour, I'd always point someone towards a mid brown or a soft tan before black, because black shows every scuff on a structured edge and a cheap black bag looks tired within weeks. A good tan ages well and hides the small knocks that come with a shape that doesn't flex. If you want something with more personality, a muted colour with a different finish, like a matte rather than a shiny, wears better day to day than a bold tone that fights with everything else you own. Patent looks sharp in the shop and shows every fingerprint by lunchtime.
Try it on with a coat on, because a structured bag that sits fine over a jumper can suddenly pull strangely once there's bulk under the strap.