Check the target supports IPv6 first
This is the whole decision. A site reachable only over IPv4 cannot be reached from an IPv6 address, no matter what you do. Confirm the target resolves over IPv6 before buying, because the cheapest proxies in the world are worthless against a target that cannot accept them.
Cost per address is unmatched
Scarcity is what makes IPv4 addresses expensive, and IPv6 has no scarcity. Where the target supports it, nothing else gets you comparable volume for the money.
Blocking behaviour depends entirely on the target
Some sites treat IPv6 exactly as they treat IPv4. Others apply blanket rules to whole IPv6 blocks precisely because they are cheap to obtain. There is no general answer, test against your specific target.
What to buy
Confirm your target answers over IPv6, then buy IPv6 for the volume. If it does not, this is not the category for you.
Common questions
Why is IPv6 so much cheaper?
Because there is no scarcity. The IPv6 address space is vast enough that allocations cost the network very little, where every IPv4 address is a contested resource with a market price. That single fact explains the whole difference, and nothing else gets you comparable volume for the money.
What is the catch with IPv6?
It only works against targets that answer over IPv6, and a great many still do not. That is one compatibility check, and it decides whether this category is useful to you at all. Run it against your real target before buying anything, because no amount of volume helps if the destination cannot be reached.
Can I choose a country with IPv6?
Not here. The IPv6 pool is United States addresses and always rotating, with no geo targeting, which is part of why it is priced the way it is. If you need to appear in a particular country, residential or datacenter is the category to look at instead.