@ILoveWifi 

My personal preference would be E4.

Based purely on WiFi specs, E4 and M3W are identical. Yet, if you could call E4 a full size Deco unit, then M3W is compact size Deco unit. The question becomes, what hardware and performance sacrifices TP-Link had to make to squeeze everything into the compact unit? 

Without clear answer from the manufacturer, I'll go with full size unit: E4.

Deco E4 has two Ethernet ports, they are 100mbps. This is what you have read about, and it is not related to wireless performance. Other Deco units, such as M4, they have gigabit ports. From what you are saying, I can conclude you'll have E4 wirelessly connected to your existent mesh system. In that configuration, Deco unit Ethernet port speed makes no difference.

Even if you do not plan to connect anything by Ethernet cable to new Deco unit today, that might change in the future. 100mbps is more than enough for devices such as Smart TV or Android set-top box. Deco E4 has two Ethernet ports, M3W has none. In that respect, E4 is better choice.