AFAIK from the trade journals the WT series is the update of the WSD series. The WSD series had growing pains for a station that is very similar in function to the Hakko 888D. The predecessor of WSD was WESD51 which was a digital version of the reliable bug free analog WES51, still made today in Mexico for the US market and was very similar to defunct Hakko 936, which the 888 series replaced.
While 888D OTAH is bug free, the 'digital 'component is unnecessarily complicated, whereas the WSD was unnecessarily complicated, buggy, and more unreliable. Replacement and consumable parts for Weller also cost more. Maybe it improved prior to its replacement or the reason for the WT redesign, I do not know.
If you are in the EU, Hakko units come at much higher prices than the US or Asia, making the Weller units more price competitive.
As P90 said, adjusting a digital scale with buttons is far slower and more involved than the simple twist analog pot it replaced. If this had some functional reason, it could be acceptable, but the digital readouts are more cosmetic than functional. Luckily the 888D has 5 'preset' memory positions, so you can store the most oft used settings, say increase by 50: 250 300 350 400 450 etc., so you can quickly go to common settings and fine adjust with the buttons as needed. Of course, you have to remember how to get to and how to set the preset
It was faster to simply turn a dial to that temperature than to press a preset.
http://www.electro-tech-online.com/threads/choosing-a-soldering-station.119211/page-3#post-1041456Avoid digital stations that require some form of calibration. It means there is no temp sensor at the tip so the readout is an approximation for the true tip temperature. To calibrate the tip, you need a thermocouple in a tip immersed with solder to measure it accurately, and this temperature is then used to adjusted or offset the station readout so its the same.
Different tip geometries change the tip temperature if the station power output is also not adjusted to match. Hakko's literature gives the approximate offset caused by such change depending on the tip you use. You can eyeball the differences or adjust the station accordingly. This is one reason in the original 936 station the markings are no better than 50C increment as there is no value in more accuracy. So any station using a similar technology is misleading when using the precision of a digital scale.
You can always put tape to mark on an analog dial some offset of the dial readings to the true temp. On a digital scale, if you don't keep it in mind, its 'accuracy' misleads the user.
Users may opt to not use the temp readouts per se but only as a general guide and simply go by feel once the temp setting is approximated. You can read about the value of a true tip temp in the eevblog archives.
Weller calls adjusting the output temperature calibration, 'offset'. Hakko FX888D calls it 'temperature adjustment'. Neither station mentions the need for a good thermocouple based thermometer to complete procedure to set the offset or adjustment. That each has such a setting means both stations require periodic calibration.
Hakko has a lower cost regulated stand alone iron called the FX600 or 601 which incorporates the 936 type technology into the handle, the difference in both models is just the size of the tips used; its the same handle otherwise. In many ways, this is what the FX888D should have been. Although the settings have detent notches, in between positions do work. The main drawback is the handpiece is heavier than a station plus pencil combination, but its completely compact and easy to transport.
https://www.eevblog.com/forum/reviews/hakko-fx-600/https://www.hakko.com/english/products/hakko_fx601.htmlEnjoy.