I haven't ever really tried to compare the DB press to a KB press. It's an interesting question. When I started training in the gym 10 months ago, I could comfortably do sets of 5 KB presses, like do a 5*5 all at 40kg with both hands in 15 minutes or so. Some time soon after starting in the gym I picked up a 40kg dumbbell and it was a bit of a struggle to get it up. I thought I'd just pick the weight up and lift it as usual and I was really surprised by the increased difficulty and naturally I haven't done something that awkward again.
Since I loaned my kettlebells away in the summer - has it really been that long? - I haven't done any KB military presses in a long time. I haven't really done a lot of overhead pressing at all in a long time. I'll get back to it come this summer and I get my kettlebells back. After I get them back I'll do a real comparison.
One thing that I wonder about is if there is a significant technical difficulty with the dumbbell press just because I haven't ever trained it. If a hypothetical person had done equal amounts of pressing with both dumbbell and kettlebell, would there still be a major weight difference between the lifts? I think in our cases we will never know.
Now that we're talking on the subject, I think dumbbell presses should be a great accessory exercise for the kettlebell presses, and that getting better with the dumbbell on the side should also help the kettlebell move better.