It depends on what you want, FX888 is soldering iron only, that 899D has an extra hot air gun, you can do 0402 more precisely than with a soldering iron.
The basic difference is that FX888 is 70W while 899D iron is 50W.
I would grab a FX888 and another hot air "only" station. I always used "all in one" stations but I think now the quality of "dedicated" stations is better. The disadvantage is the space it will take on your workbench.