Hey, Miska,
The two strollers are really very very comparable - similar in size (folded, too), similar in space... it's not totally easy to tell which is "better" as both will have the same pros as well as cons, pretty much. I personally would probably lean towards Koelstra as that one is a little less slanted in the siting position - the Bonny is leaning backward a bit too much, making the baby only half-sit. I also overall like Koelstra desgns, they are usually practical.
Otherwise, whichever is prettier or cheaper for you will be truly almost the same, functionality-wise. Just be prepared for very small, foam wheels, thus not much terrain capability (these are meant for use as a secondary stroller, for traveling, shops, and so on).