Breast plates and martingales are not the same item -although some breast plates also have an optional martingale attachment.
What problem are you trying to solve and why? The different breast plate designs have different affects. And for that matter fit differently. A racing breastplate fits to the girth only and stops the saddle slipping backwards, but not tilting. A 3 point breastplate fits to the saddle D rings only and will prevent tipping. (As well as giving a convenient grab point for the rider!) A 5 point fits to both girth and D rings. As well as what security you are trying to find for your saddle you need to consider the chest shape of your horse - some do much better in one design than another. Most makes of 3 and 5 point breastplates come with a running martingale attachment; but you would need a separate martingale if you want a standing, irish or bib. Racing breastplates don't have space for a martingale attachment but usually have a slot in the front you can feed a separate martingale through.
If you can give some more detail on what you are trying to solve and your horse type it would help.
Hi, thanks for responding, I should have elaborated, sorry. My childs instructor has suggested we use one more for a 'grab point' and extra security rather than because the saddle is tipping or slipping. Our pony is 14.2 and isn't a slim jim but not a chunky cob, although he does have some substance. He doesn't toss his head up but is a jumping pony and can get a little excited.
Buy a Monkey Grab or Grip Strap if it is just a grab handle that you are wanting. They fasten onto the front D rings and are just that - a grab handle, but only about £10.
if your instructor has recommended a martingale then you can buy a perfectly decent kincade one for about £20. I would actually say a plain martingale will be easier for your child to grab than the breastplate version as with these the grab point would be much closer to the front of the saddle. Id also say the same for a grab handle. If your child is jumping and needs to grab something his/her hands will hopefully already be giving up the neck and so a martingale neck strap is in a better place - or if an actual martingale isnt need then a plain neck strap might do the job.