So much of this comes down to the instructors. If those are your two options and you are looking to use them to get your foot in the door for MMA you need to have a heart-to-heart with the instructors to find out their goals for their students and see if they align with your own.
From my experiences I would personally lean towards kickboxing unless the Karate instructor really impresses you. I recommend skipping any style that disallows punches to the face in competition as some things are very hard to unlearn.
Another quick check is to look at the physical condition of the other students; it often speaks volumes as to how recreational the setting is compared to your interests in MMA.