This was an interesting question about whether you should have a mixer plus a separate recorder to go into or an all in one unit. Personally with the Sound Devices 688 I love the all in one solution but its important to point out this key difference when looking at these solutions:
Is it more a mixer or more recorder?
This is an important question to answer for the line of work you want to go in for. An all in one unit will not work for a feature film because your mixer faders will be too hard to operate on an all in one solution. Essentially is it more important to have the qualities of a good mixer with a recording function or more routing and recording qualities and put a mixer in front.
This answer will change drastically depending on your line of work but for me I can have the all in one unit solution a then if i need to expand i can put a mixer in front to interface with it as a recorder.