IMO sling boots are better than splint boots. Splint boots only protect the canon bone whereas sling boots wrap under the fetlock to fully support the joint.
I don't think you can get a better pair of boots than the slings. I just like the fact that they really support the whole lot, not just part.
It won't kill your horse to not have boots of any kind for this one race. I do however suggest in at least trying to borrow some splint or sling boots for it and then invest in a pair for yourself.
I got these ones for my horse Yimar Neoprene Sling Horse Boots MEDIUM P/W HSP2 - eBay Boots, Clothing, Horse Riding, Sport. (end time 31-Aug-10 19:00:58 AEST)
They were only $33 including postage and the inside is double reinforced. I love them. I only got them a week ago and could only use them once before my horse went lame, but in the 2 rides that I had with him before that, compared to riding him in my splint boots (which also streched btw) he felt a lot better getting that extra bit of support.
They were also really good on postage. Ordered them on a Thursday, got them in the mail on the Monday after.
Not to mention they look awesome. They also have blue velcro ones