My first binder was a size small tri-top. I'm about a 34-36 B in bra sizing. It gets me flat enough to pass, decently flat with only one shirt over it. I usually wear my binder, then a white t-shirt, then whatever shirt I'm wearing and it usually gets rid of the top hem of the binder showing. It dug in at my armpits for a while, but over time got better. It didn't roll up on me unless I ate a big meal, then it was kind of uncomfortable. I had it for about a year and it finally tore down a seam. 🙁 I liked it better than my double front though in terms of comfort, because I didn't have to worry about tucking it in and it didn't roll nearly as bad. I put this binder on by stepping into it with the straps on top and just pulling it up.
My binder now is a size small double front. It was slightly used so it was a little stretched out when I got it, but not much. I'd say it gets me as flat as my tri-top did. I get the same arm pit problem with this one, and it hasn't gone away yet. I really hate the bulges it makes when my fat sticks out my arm pits! Sometimes the front hem sticks out more than my tri top did, but I've found that if I re-adjust and pull the binder up higher, it's fixed. I put this one on by stepping into it with the bottom hem facing up and the binder inside out, then when the bottom hem is even with my hips, I pull the straps up. This one is a little more uncomfortable for me than the tri-top because I have giant hips. I wear compression shorts and tuck my binder into them, but it never stays put very long. It likes to roll up pretty often. I like the hip compression it gives me, I just wish I could find a way to get it to stay down. Any ideas?