FYI: my FX-888 (not D!) is rather useless at 300C, and I default to 350-400C for leaded soldering on normal to heavy pads.
The heater geometry is a ceramic hot-end that fits inside the tip. Conduction is through an air gap. The responsiveness is good, but far from perfect.
Tip wear is low (using brand name tips anyway), and oxidation is modest (okay to leave at this temperature for tens of minutes). I do try to turn it down when not in active use (okay for hours, days? at 250C). This is rather awkward to do on the 888D's (or did they add an idle mode, I don't recall?). For this one reason (control knob), I would recommend WESD-51 over FX-888D, for those of you that happen to be in the market.
A higher end tool, using cartridge type tips with integrated temperature feedback, will be able to run at a lower temperature, because that temperature is more representative of the tip itself. Higher end Hakko, Weller, most JBC (all?), etc. are this way. Also Metcal and clones, where the tip is regulated by Curie temperature (in effect: excess RF power is reflected back to the power supply depending on tip temperature).
So keep that in mind; it depends on the tool.
Tim