Here's the perspective of one rider that started with traditional dressage and then stumbled onto western dressage.
Our western dressage debut in 2018
Tack, obviously. That freaking 35 pound saddle is a dead giveaway as to what style I'm working on. Horns are not required on the saddle but western style fenders are. There's also very different rules regarding bits. My loose ring snaffle is allowed for both disciplines, but a curb is allowed at all levels in western dressage. Ports of up to 3.5" are allowed as are bitless bridles, bosals, and spade bits. You see quite a variety of rigs, though the hands down most common is a small Myler curb bit. Even Theo owns one. For some riders having more bitting options is a reason to go western but I've heard rumor that the rules will change to snaffles only at the lower levels in the future.
Short shank Myler bit, good occasional reminder when pony decides he wants to hang on my defective shoulder
Two hands are allowed at all levels and with all bits, but one hand is also allowed with a curb bit. Two reins (a bit and a bosalita) is also allowed and I'm tempted to train Theo to a bosal with the goal of going two rein. Probably this winter to keep us from going crazy in the indoor. One day, in the distant future, I want to ride a test one handed. It's a good goal to work toward.
The movements are a bit different between the two disciplines. Turn on the forehand is seen in competition starting at Level 1 and keeps showing up right through Level 4. Turn on the haunch is a stationary movement and goes up to 360 degrees. Want to work on your horse's balance and attention? Do a 360 degree turn on the forehand right into a 360 degree pivot on the haunches. There are no mediums or extensions, just lengthening of stride in jog or lope. Not all tests have lengthens or might only lengthen one gait. A lot of movements hint at reining patterns. No flying changes until Level 4, but you also get half pirouettes and a canter entrance at that level so it's a big jump from Level 3. There's more halts in general and a lot more reinback, including reinback on centerline. Yikes.
Level 4 Test 4, currently the highest level test
You can talk to your horse and cluck at them, which I love.
Jog and lope are, in reality, not that big of a change from trot and canter. Especially with the collected gaits, I change very little between the two rings. The neck still lifts, the hocks step under and take more weight, all of that. I turn the dial up more for the traditional dressage ring so that he shows a bigger trot and canter. In western dressage, Theo's natural collected gaits are quite good and I let him just do his thing. Not having to turn the dial up keeps him in happy, floppy eared pony land and keeps our harmony score up. It's most definitely not western pleasure gaits but don't expect to get great scores just because of big gaits. If it doesn't look easy to ride, looks tense, or feels frantic, it's not going to score well. Think about the kind of gaits you'd want to live with if you needed to spend hours in the saddle or the gaits you see in ranch pleasure classes. You want to get there in a timely fashion, but it needs to be something you can live with all day. Your horse also needs to look like they can keep it up for awhile.
Doing a good job of looking very chill
The biggest difference in my experience is in the expectations and the scoring. The emphasis is on a horse that looks obedient, attentive, and easy to ride. Light contact, light leg, harmonious picture. Heavy contact is a big no no. Instead of a submission score, it's a harmony score with a coefficient of 2. Harmony is usually our best mark. When the judge says your horse looks like a blast to ride, you know you just got a good score. You're being scored on how easily and smoothly you as a team complete the test. Your goal is to make your horse look like a super fun, easy ride that the judge wants to kidnap for their own barn.
Not really all that different
In traditional dressage, I get hit a lot for Theo not having big enough gaits or enough energy. Those requirements change in western dressage. Your horse must be forward thinking but not at the expense of rideability. Your horse needs to be uphill in collection but it can't look tense. I still get 'needs more ground cover' in my lengthens, so that doesn't change. A horse that doesn't halt well or jigs is going to have a hard time. Halts are everything in western dressage since you're doing it all. the. time. Also straight lines away from the rail and down centerline. The Level 3 Test 4 with the canter-halt-turn on haunches-canter sequence is brutal if your horse drifts. Spoiler: Theo drifts.
This is the old Level 3 Test 4 but you get the idea, scored 76.7%. This was also our judge at our last show
Western dressage is getting more popular. It seems to hit that niche for riders that find traditional dressage to be out of reach for them or their horse but still want to focus on the precision that the sandbox requires. Several of the more nervous ladies I know feel much more comfortable presenting at the jog and lope in a western saddle. Less emphasis on 'get them more forward' and more saddle for them to grab on. Showing in jeans? Sign them up! And the pretty saddle blankets. They own QHs and paints because that type of mind makes them comfortable. Now they have a chance to show them in a venue where they feel comfortable and those wonderful minds are rewarded.
For Theo, western dressage is the better fit. It matches his personality and natural abilities. We will still show traditional dressage because, I want that dang Bronze. I also find that the traditional dressage training pushes us to improve his power and forward thinking so my western dressage tests look easier and easier.
If you've got access to a western saddle and there's a western dressage show, it's fun to try. Some horses do very well with the change in emphasis. It's new movements for a seasoned competitor to learn and you get to show in jeans. Seriously, showing in jeans is amazing. White breeches are dumb, give me dark wash jeans any day.