Find a saddle shop local to you and sit in a BUNCH of saddles. Find what feels comfortable to you before deciding what to go with if you don’t have a discipline in mind. I do ranch versatility and could show in anything. I use a ranch roper with a Will James style tree because I like the deep pocket and the ability to rope and drag just about anything I would ever need to. A ranch cutter on the same tree (just not as beefed up with rawhide and a slightly narrower bar) would suit my body shape just as well, but wouldn’t offer as much on the roping side of things. I do not find the shape of a reining seat comfortable, while they are deep, the rise at the front doesn’t agree with me. I find cutters too flat and too narrow for my hips. Barrel saddles come in many different seat shapes and some fit me fine, while some place my leg too far forward and I’m constantly fighting to keep them underneath me. Additionally, I find the tall horn gets in the way while I’m riding. Roping saddles are hit or miss, I prefer a higher cantle and most don’t offer that, I’m not particular about swells. Sit in both hard and padded seats. Don’t knock the hard seat until you try it. Most think it’s uncomfortable and won’t ever give it a shot, but I’m several years in to hard seat saddles and padding is actually less comfortable. Do everything from team sorting to hours and hours of trails in my hard seat saddles and I do not get sore. I’ll get sore in a padded seat after an hour or so. Give the hard seats a chance even if you think you won’t like it.
Sit in everything you can, it should be comfortable before you even consider buying it. Don’t fall for the ‘it’ll feel better once I get it broken in’ that doesn’t work. If it pinches or puts pressure somewhere you don’t like, that won’t change. Take your horse with you to be fitted if you can, western saddles are a little tougher to fit on your own. I work for a saddlery and try to sit in EVERY single saddle that comes through our doors, new or used. The more you sit in, the better your feel will get and you’ll know pretty quick what fits you and what doesn’t. After you consider disciplines, consider what you might grow in to and get a saddle to accommodate. If you think you’ll throw a rope at some point, don’t get a barrel saddle or cutter, they can’t handle roping. Trail saddles are a great all around saddle, some may be able to handle a rope or two, some can’t.
As for differences in type -
Ropers - will be stouter built, heavier, and made to take a beating. Any seat shape can be found. You’ll see all sorts of horns on them, but the bigger the stronger they tend to be. Swells differ on these as well. There are many types of roping and a saddle for each type. Sub type Wade trees or slick fork saddles (big horn, no swell. Popular buckaroo saddle, often used by working cowboys) They’ll come in hard or padded seats, slick or roughout leather, the latter being more common.
Cutters - High swells, tall and thin horn, and flat seat. Bars will have a sharper twist to them to help the rider grip better through quick changes in direction, feels narrower to the rider, not great for those with wide hips. Most tend to have a lower cantle and be a hard, slick seat. You will find padded seats and roughout cutters too though.
Barrel - High swell, tall and thin horn, deep seat, forward hung rigging, and a tall cantle. These are meant to hold you in and will typically have a padded seat and be roughout. There are, or course, exceptions.
Trail - Heavily padded, slick typically, taller horn (but thicker than barrel or cutting), mid height cantle, swells vary from average to slick fork. These will almost always have strings or d rings all over the place for tying on saddle bags and whatever else you need to pack. Popular choice among pleasure riders/weekend warriors.
Pleasure/Show - BLING. All the silver. They come in just about any shape, not typically used as an all around type.
Plantation - These aren’t necessarily ‘western’ but have horns so let’s roll with it. They are typically advertised for the gaited horses and have an extra flair on the bars at the shoulder for ‘enhanced’ gaiting and shoulder movement. They will often look like stripped down (minimal leather, skirts, etc.) trail saddles. Padded and slick typically.
There are many more, but go check out a store with a ton of saddles. Sit in stuff. Nobody else can tell you what will fit you. Let your butt decide.