The problem with both these games is that neither play like F-Zero, those perfect and precise controls are nowhere to be found here. Redout plays out like a Wipeout imo, be ready for float controls. Fast Neo is somewhat in between Wipeout and F-Zero, no floaty nonsense but there's still a distinct lack of total control over your car.
I guess the primary choice you have to make is for or against floaty controls. Both games are novel but got stale surprisingly fast for me, way faster than any F-Zero game or even Wipeout games which I'm not even a fan of. I'd rather skip both of these and wait for the never happening F-Zero GX remake, or even another F-Zero X VC release. If I had to choose I'd pick Fast RMX because at least it feels more unique and is more visually pleasing in a speeding kind of way.