Nice deal you've got on eBay 
Per CPU spec: Adtran SDG-8733 should outperform W1700K in wired routing due to a faster CPU. It appears Adtran SDG-8733 is also using MT7996AV as its WLAN hardware, which is the same WLAN hardware found in W1700K. So wireless performance could be on par. The final difference is their NPUs - which may need some specific testing to find out which one gets the edge.
You can check out the long thread on W1700K for more detail about it. The upside for it is you got a lot more testers due to many people getting it for very low cost.