There are people who say that an alloy steerer loses a lot of the vibration damping of a carbon fork. Like the tube shape (notice I said "bike brands claim..."), it's probably a relatively small effect, particularly on a wide tired bike. So, probably mostly weight -- about 250 grams is a typical difference.
If someone who has actually used that EBB says it works well, then I'd go with that. I'm just speculating on the design from the pictures.
An eccentric hub like the White Ind ENO will make any bike a singlespeed. The biggest drawback there is that it limits you to singlespeed freewheels. With a frame like the Thunderbird you can use whatever multi-geared rear hub you like with a single cog and spacers, which I think makes gearing changes a bit easier. The disc version of that hub also looks like a bit of a headache.
I think you're right that for $529 the State Thunderbird doesn't look bad at all. A point of comparison would be something like a Kona Jake the Snake frameset, which I think hits all the right points in the areas I mentioned but costs about $700 and you'd still need to buy your own headset and eccentric bottom bracket -- so around $900 to get to where the Thunderbird starts.