Toptank is top-filling, you just unscrew a topcap, fill 'er up, and replace. Subtank is bottom-filling, you have to take the tank off your device, unscrew the base (with juicy coil attached), set it down somewhere the
juice won't make a mess, fill up the tank, replace the base-and-coil assembly, turn it back right-side-up, reattach it to your device, and then clean up the
juice that the coil left wherever you had set it down before.
Also, the Subtank comes with OCC heads, while the Toptank comes with the new SSOCC ones. I do believe you can use either one in either tank, though, when it comes time to buy replacements.