Coldfire:
You will be displeased with neither the 510, or RuyanDirect. I recommend grabbing some 901 cartridges, and a DC charger while your at it. You have to sand the 901 cart's down just a bit to get them to fit, but the whistle tip is much more comfortable.
You will find that you need a USB Passthrough soon. Check out CigTechs.com, and ElectroNicStix.com for US suppliers (and fantastic customer service!!!).
There is a thread on coupon codes here on ECF for up to 15% off. Check it out!