Where do you buy these in the US?
There are two official distributors to the West. One is Beini, a core member of PenBBS, and she sells direct from China over official Etsy and eBay shops. (There are various other unofficial resellers from China. Etsy, eBay, and AliExpress all have regular resellers.)
The other (confirmed on Beini's Instagram account) is Gold Spot Pens. Stock is what it is, though. They don't do huge production runs, and have to split them between their official Chinese and western distributors.
In the for-what-it's-worth category, the 456 is my favourite pen, but I've been using the 355 recently as an inexhaustable workhorse. They have the same nib units (though there has been more variety of nibs on the 355 in the past), so the writing experience is largely the same.
The filling system on the 355 does take a while to get used to, so I don't recommend getting an opaque colour if it's going to be the only one, and definitely practise with water the first time. The 456 is straight forward.
If you're a tinkerer, there's an easy hack of shaving the finial screw of the 355 a few millimetres to get longer nibs in it. I had a Bock B on one that didn't fit the first time I tried, but seating it deeper in the nib collar got it to work without shaving down the finial screw.