Spitfire Symphonic Motions and Kepler Orchestra both provide rhythmic solutions for composing, but there are some significant differences listed below:
Symphonic Motions is a dedicated plugin which loads directly in your DAW, whereas Kepler Orchestra is a Kontakt Player library, meaning that it is loaded inside of the Kontakt Player Plugin.
Symphonic Motions is purely a String library whereas Kepler Orchestra contains brass and woodwinds, as well as more synthetic sounding "Warps".
Rhythms and techniques
Symphonic Motions has some of the most commonly found rhythmic values such as 6th, 8th, 12th, and 16th notes played both discretely and smoothly across a number of techniques whereas Kepler contains several more esoteric note values such as tuplets dividing the bar into 5 and 7. Additionally Kepler Orchestra contains various other techniques such as shards, dopplers, and momentum patterns (more info here)
Symphonic Motions is was recorded in AIR Lyndhurst Hall, whereas Kepler Orchestra was recorded in the smaller, and drier, AIR Studio One.