I have used the dressage sport boots and the dover boots. I like the dressage sport boots a little better as they are more flexible and seem to hold up better. I would recommend measuring the circumference of your horses cannon bone and calling who ever caries the boot that you want to check size before you order so you don’t have to return the boots for a different size. a medium for a 17 hand warmblood sounds too small to me. My 16.3 and 16.2 hand warmbloods both wear large boots in any kind I’ve tried. In fact the only warmblood I have ever had who has worn a medium is my currently tiny 4 year old who is 15.3 hands and I don’t think she is going to stay this size.
I have tried the professional choice boots and I don’t think they actually are as cool on the legs as the fluff lined boots (actually real sheepskin is the coolest, but a pain to take care of and expensive). I think all of the plastic boots get hot and I would not use a neoprene boot for that reason. I make sure that I put boots on right before I ride and I take them off as soon as I get off as well.
Another thought: why do you want white boots for schooling? I only order black or grey for schooling as white never stays white. If you want white for show warm-up you can use restrict their use so they stay white longer. Otherwise they get dingy looking pretty fast.