Hi Vicky,
The difference between a flat and convex mirror is the "viewing angle" and the convex mirror gives you a "wide angle" view of the road behind.
When you fit a convex towing mirror to your car, you 'll see a much wider view of the road behind the you; so it doesn't matter quite so much about aligning the mirror, but it's best if you do, and then you'll be able to see much more behind you.
The downside of the convex mirror is that vehicles behind you will look much smaller (and consequently further away) than they do in a flat mirror. You need to get used to this effect and make allowances before manoeuvring.
If your car door mirror has a warning message engraved on the glass or a line of vertical dots down part of the glass - then all or a section of the mirror will be convex - and you'll already have an idea of how much more you can see and how much smaller the cars are.