I have used both, and used both on large accounts. I would 100% go with marin. Kenshoo is very slow when syncing large accounts to the adserver api, and doesn't let you push changes to alternate channels if one channel is being sync'd (i.e., you have to wait till bing is done syncing before syncing anything to google). I also found kenshoo harder to navigate - their advanced search feature is nice, but I prefer the filtering in marin. I also prefer the dimensions in marin. Kenshoo has a few more bells and whistles in the bidding department as far as 'predictive technology' goes (supposedly), but with all the other drawbacks (plus, marin has a great a/b creative testing suite AND plenty of ways to auto bid), I would go with marin for sure.