The FX-950 is only about 250 HKD more expensive than the FX-888D. I'm definitely willing to pay that difference for a knob control.
If you're willing to forgo the knob, the FX-951 offers additional features, particularly regarding the setback features. Specifically, the FX-951's stand is connected to the station (unlike the FX-950), letting it know when the iron has been placed in the stand. This allows the user to set how long it takes for the iron to either drop to a settable lower temp, or shut the iron off. Now if you set a temp for the setback when the iron is initially placed in the stand, there's a second setting that will shut the station off if it exceeds the set IDLE time*. All of this can ultimately reduce the amount of oxidation, if not eliminate it, depending on both settings and tip care.
Granted, the UI isn't the easiest to use, but once you've got it set, it shouldn't be that big a deal based on what I've read from posts from owners (set it, then leave it alone other than perhaps switching between preset temps, which isn't that difficult at all once they're set).
* This might not seem like that big of a deal, but it can significantly extend the life of your tips (can make a huge difference in TCO).Not sure of the price difference in you're particular market, but it's worth digging into to see if the features are worth it IMHO.
Also, Hakko's tip plating is one of the thickest out there, if not the thickest (not sure of the thickness vs. Ersa's tips, but it beats recent Weller production IME for example). Ideally, the plating thickness would need to be compared to both Metcal and Pace IMHO (suspect it would beat Mecal; not entirely sure about Pace, but Pace isn't so easy to get vs. other well known brands, particularly internationally). Regardless, Hakko's tips are well known to last a long time. For example, using 63/37 with a flux that's suited to electronics soldering (i.e. R, RMA, RA, or a suitable no-clean within these levels of activity), as well as cleaned + tinned, it's not unheard of for tips to last 10+ years.
Seriously, they're that well plated.