Don't worry much about the plastic, it's quite durable I only managed to snap off the rear cable clip guide by force during stressful cable management
Noise is only if I put the fan to high 1800 rpm (still bearable) or manually blast 2500+ rpm while stressing all the four cores doing compiling or software transcoding (hardware transcoding it runs cool). At auto, it spins at around 500 rpm you can't even notice and it should be similar to the AS6404T.
Heat is also not an issue. I'm glad it's aluminum inside but not on the outside as it's not too hot to touch compared to aluminum casings. Some aluminum casings have good heat dissipation but hot weather can quickly influence the temperature inside.
The rest of the specs depends on your preference as seen on the comparison table. I would prefer better CPU twice the cache, RAM more the better I'm running 16GB not sure though AS6404T supports more than 8GB. SPDIF? I'm using HDMI passthrough. 4K? Both are capable but the AS5304T was advertised for HEVC 10 bit and I've lots of these videos. USBs? I'm happy with the average write speed, around 70MB/s per each external 3.5" on raid 5 (average 200MB/s) and slightly higher average of 250MB/s for a single SSD. Reads are of course higher but I noticed the CPU have most impact on the USB speeds. 2x2.5Gbps LAN? Ok maybe I do use the adaptive load balancing but I'm still on 1Gbps network.
Aesthetically IMHO AS6404T is targeted for business but I don't find much value in its LCD (I might be wrong just because I don't always look at it) and the AS5304T slight performance is targeted for home gamers LOL (played CS:GO at its lowest settings + 720p).
