If it was my personal choice it would be neither but if it was between the two then it would be Lenny every day of the week for me.
Martinez has some of the best footballers in the world at his disposal as Belgium manager so I don't see how his achievements on that front translate to him being a good fit at Celtic and although he was decent at Wigan he didn't exactly set the heather on fire at Everton. As for him being high profile/big name. Really?
Martinez is NOT a step up from Lenny imo so what would be the point?
As for John Kennedy being a future Celtic manager? Based on what?
If JK left Celtic and became a manager in his own right, worked his way up the ladder in management and was successful then sure he may become a viable candidate as Celtic manager sometime in the future but until that happens he shouldn't be anywhere near the job.
Having a 'Celtic man' as manager should be seen as an added bonus but it certainly shouldn't be a requirement.