any of these devices will connect to an external dac but i wouldn't connect straight from any of these devices to an external dac without going thru a jitter reduction/eliminator device. apple tv gives you much more functionality than the squeezebox (video), but for audio only, both units give you the option of using analog outputs to a receiver/preamp or using the toslink to a jitter device/dac and the squeezebox adds a coax output which is normally preferred connection if going straight to an external dac.
apple tv allows wireless 802.11n and the squeezebox uses 802.11g, the apple tv can network up to 300mbps vs squeezebox 54mbps, and the wireless coverage using 802.11n is much wider.