which one of these shoes would be better suited for the 400/800m runner?
What level of athlete? I don’t really like the Nike Rivals, but there is nothing inherently bad with them, just they seem to wear out quickly. I have heard that Adidas spikes can be a bit too stiff, but that may not be the case for a middle distance spike like that one.
The level is a high school athlete. Does anyone think either of the shoes would be beneficial or are there other shoes that would be better?
This is all a matter of personal preference. It’s best if you can try them on to see how they fit and feel. Typically, you want a shoe that is flexible and with a slight heel. In my experience, and in seeing other posts on this site, mizuno and asics are preferred, but there are some nike and adidas fans as well. With so many brands and models within brands, it’s hard to pick one or two as best. You might want to check out some mizuno tokyos or mizuno geo snipers. They have some good closeout prices of those at dickpondathletics.com
I’ve tried some nike maxcats, which were light and flexible but the spike placement bothered me a bit. My advice is to try them on before ordering – especially if you’re paying full price. You might try gambling with some lower cost closeout models.