IPv4 or IPv6 proxies
The sixth version is cheaper and there is more of it. That is where most comparisons stop, and it is the wrong place to stop: plenty of platforms will not take it at all.
The difference that decides it
Every platform accepts IPv4. Not every platform accepts IPv6, and that is an ordinary situation you should plan for.
When the receiving side does not support the sixth version, the request never arrives. The run stops at the first call, and the cause looks mysterious: the addresses are alive, the collector is running, no answers come back.
The quieter quality gap
On some platforms the sixth version is accepted but served on a best-effort basis: higher latency, a visibly larger share of timeouts.
Over a long run that eats the price advantage. Paying half as much for addresses while the run takes half again as long is not a saving.
A one-minute test
Send a single request to your target platform from an address of each version and compare the answers. An empty body or a dropped connection means the version is not supported.
Test against the platform you will actually work with. A generic IP-check service tells you nothing about this.
What to take for what
For search results, catalogue sweeps and price collection, take the fourth version: it goes everywhere.
Leave the sixth for cases where the receiving side is known to be modern, or for internal transfers where both ends are yours.
curl -4 -s -o /dev/null -w "IPv4: %{http_code} %{time_total}\n" https://example.org/
curl -6 -s -o /dev/null -w "IPv6: %{http_code} %{time_total}\n" https://example.org/