The spyder has more value and will always have more value. Rarity doesn't directly translate to value, and these days any clean H body is a rare car. But spyder gets the nod due to graphics and a cool name.
I do think the sunbird would have equal value if H bodies readily accepted pontiac V8s. There'd be enough trickle down from the pontiac faithful, but the thought of chevy powered pontiac turns those guys off. Imagine a world where you can bolt together a super duty 455 sunbird the way you can do a chevy V8 today
In the world of higher dollar restorations, modifications like disc brakes don't seem to harm value as long as the car is a legitimate road car, and the modification is bolt-on. Plus with an H body, 5 lug is a huge plus as it opens a world of wheel choices. I would say most buyers will see that as a positive and it'd gain value despite losing originality
good luck with the cars