My take on the ADS200 vs the FX951 is that it comes down to personal preference. I have used both irons and personally, I like the ADS200 more but they are both fantastic Irons and worthy upgrades to the FX888. In my opinion, the tips for the ADS200 are better but again, Its really up to personal preference.
Pace also has currently one of the best mini tweezers on the market, so when the new ADS200 aluminum tweezers are finally out they will have almost the same power as the FX951 iron but on each "tweeze".
Neither stations are perfect, just the ADS200 solved a lot of the nuances the last generation of stations had which happened to include the FX951. I resisted temptation for years on a FX951, fortunately the price over here helped with that. To be fair Hakko is still the best thing going at $100 US, just not the best thing going at $200 US anymore due to the ADS200.
Paces design for a while has been very simple, I like em. I was initially looking at the Metcal, Thermaltronics and Paces older iron came up in that. When you do a comparison the Hakko starts to look a bit clunky with it's dependence on a tip holder. I'm fairly sure this was done to avoid heat issues which thankfully Pace has solved for their own stations.
Not that Pace have said this but I think the reason they went blue is they planned to replace their older plastic iron (top right) with the near identical looking silver anodized version soon after the ADS200 came out. Been soldering almost 40 years and aluminum with contoured silicon grip is a great combination if not the best I've come across.