As I understand it barrel grips tend to be more popular in Europe.
I spent a long time handling the Bosch 1590 and 1591 before deciding between barrel vs. top handle.
In the end I bought the 1590 (top handle) mostly b/c it was more readily available, and I could find it for about $30 cheaper.
Barrel grip gets your hand a lot closer to the work, some people feel like it gives them better control. That's the biggest (only?) differnce that I am aware of.
I guess top handles may give you a little more assurance you're not going to loose your grip or drop it since your finger's can wrap all the way around the top handle for control.
One of the reasons I leaned away from the barrel grip is I worried if my hands got sweaty I might have trouble holding onto what is essentially a 3" diameter plastic cylinder with some rubber glued to it.
In the case of the Bosch, they are the exact same machine, the top handle has a little more plastic to hold on to, but the internals are the same.
I grab my 1590 and hold it like it is a barrel grip sometimes, it's not as comfortable as the barrel grip would be, but for precision work while holding the saw upside down (coping trim), it feels more controlled to me than using the top handle.
At the end of the day, it's just a Ford vs. Chevy personal preference kind of thing.