I am trying to decide between two Netgear routers.

[agustusin]

Costco has the NETGEAR Nighthawk 6-Stream AX4300 for $130, down from $200 right now.

It says it's wifi six but the details show up to 802.11 ac

Is this some sort of strange advertising issue?

I'm looking at another similar wifi router from the same line that's newer and more expensive.

Netgear Nighthawk AX4200, which appears to be a true wifi 6 router with 802.11ax.

[plemans]

do you have actual model numbers? 

AX4300 and AX4200 are speeds, not models. 

(and hint, if its AX in the name, its a wifi 6 device)

With the model numbers, you can look up the product data sheets from netgear to find out their actual specs. It wouldn't be the first retailer with the wrong data on their site. 

There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.