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As they both cost the same, I would get the ATD, due to its capability of connecting USB and Firewire and Ethernet devices/cables. Does that answer your "why"?
 
OK then, so basically I don't need to connect ethernet to my MBP, when I'll be on ATD?
And when I'll disconnect it from ATD, It will go on my Wifi? :)
 
OK then, so basically I don't need to connect ethernet to my MBP, when I'll be on ATD?
And when I'll disconnect it from ATD, It will go on my Wifi? :)

If you connected an Ethernet cable to the ATD and have WiFi on, yes, most probably, but even when I connect my Macs via Ethernet and both have WiFi on and are on the same WiFi network, sometimes a file transfer is via Ethernet (Gigabit, thus faster), but it also uses WiFi sometimes, I don't know why, and I have to disconnect one device from the WiFi network to properly transfer a file faster.
 
If you connected an Ethernet cable to the ATD and have WiFi on, yes, most probably, but even when I connect my Macs via Ethernet and both have WiFi on and are on the same WiFi network, sometimes a file transfer is via Ethernet (Gigabit, thus faster), but it also uses WiFi sometimes, I don't know why, and I have to disconnect one device from the WiFi network to properly transfer a file faster.

Set the Service Order in Network Preferences.
 
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