I like my Professiona Choice SMB's. I really want the newest model that they have out now. What SMB's are designed to do, is absorb the shock that our horses legs/joints naturally take during strenuous work out's - such as Reining, Barrel Racing, Gymkanna, Jumping.
The only time I put my SMB's on my guys legs, is when we jump to help preserve his legs. Think about how much trama those front legs take when they land off of a jump, and no matter how well conditioned those legs are, they are still going to weaken every land regardless because of the shock.
This is why many (not all) top level jumpers usually only jump their GP horses once a week.
Anyways, the question is: What is it that you are going to be doing *work wise* that will require boots such as SMB's?
If you are doing high end activities as already mentioned, then I can answer yes, go for it. If you are doing low level activities such as just flat work, trail riding - then no. Go with just basic Splint or Brushing Boots.
Most horses take horse size, or "mediums"; if your horse is tall (over 16h) then she might need "Large". The sizing depends on the boot manufacturer. Pro Choice are good, Pro-Equine are good, ThinLine has some support boots that are good, pegasus air boots are supposed to be good, as well as Cavallo's version of the sport boot. Just don't go for the bargain boots that are really cheap (under $40). Those never last and don't really do much of anything.
I also agree with MIE, I never use sport boots (or any boots) as I only do flat work, trails, dressage, and ocassional little jumps. I only ever use splits or polos on horses that are either young and clumsy or interfere (hit their legs together on ocassion).
Another kind of boot is the Thinline Cobra SMB. I've never used it before but I have heard awesome reviews. I personally have their open front boots which are the best boots I've ever used. The thinline lining is so much nicer than neoprene. I love that the thinline has antifungal agents and how easy they are to clean.
Luvs, what about the splint ones? Is it OK to go with the pair for $20 or they also do nothing?
Well, I plan on start jumping one of them and do come cutting and/or reining (depending on what she/I like the best) on my other one. Do I have to put them on all 4 legs or just front ones?
On trail rides I don't use such. :) I use Easyboots (if the park is rocky) or nothing. I also have a pair of splint(?) boots I'll be using for the flat work (mostly to prevent from hitting on lateral movement which she just started to learn).
The split type boots are fine cheap. All they do is protect the outer leg from bumps and rubs. They do not provide any "support" to the tendons or muscles. Though, it can be argued that the sport type boots don't provide much support either . Not in the case of a 1,000+ lb animal impacting the ground that is.