Having tried both portable transcribers and computer based transcribers, I think the computer based work best. *But if you want a portable, I'm pleased with the TR-1000 made by Reed-Kotler: *
http://www.reedkotler.com/
It's really only good at slowing things down to about 1/4 - 1/8 speed. *Any slower and it's difficult to hear the notes clearly. *You can plug in a cd or tape player and your guitar at the same time and listen to them together through headphones (which is great if volume is a concern). *It's pretty easy to setup and use. *The main downside for me is it's best recording rate is only half of cd-quality.
Do not bother with the Sabine transcriber. *I tried two of them, and neither of them worked as advertised.
I haven't heard anything about the Akai. *Whatever you decide, make sure you can return it if it doesn't work for you.