The issue with the AC Ryan box and the Netgear is that although they both probably run Linux they are essentially closed systems that aren't supposed to be tampered with..you will get a basic web interface for configuration and that is it.
So later on if you want to run say rsync you are stuck, or you want it to be your MTA as well..you are stuck, or perhaps hang your printer off it and use it as a print server too...guess what ?
Or like me with my Buffalo Linkstation, fighting up against the slightly buggy samba version it runs.
Even if someone has released an open distribution or hacked firmware for them, you will probably find they have a fairly mediocre CPU/RAM/flash. Also the AC Ryan box is only 10/100. I'd be going for something that is Gig-E in case you decide to upgrade your switches at some point.
So that leaves the sheeva plug vs the Acer Revo. This isn't as clear cut...The sheeva is fanless and consumes less power. With the Revo you are powering things like the nvidia gfx chipset and wireless which you don't need and ARM chips are just more efficient than even the Atom. But the Revo has more potential for re-purposing I guess, should your needs change.
Also the Revo was really designed to be a mediaPC so I am not sure if it supports things like autopower on after a power cut...I'll check on my one later.
All that said, there is another issue which is tidyness. With the two out of the box nas solutions you will have one box and two cables, network and power.
The sheva plug is going to need two wall sockets one for itself and one for an external HDD, plus network, plus usb cables and the HDD will be using a power brick so more clutter there. With the Acer it gets even slightly worse as you have the Acer plus the HDD enclosure, 2 power bricks and the network/usb cables.