Normally, I'd point you to powered monitors because usually, the components of powered monitors (amp and speakers) are designed to work optimally with each other.
However, the
BM5 is a higher quality speaker than the Behringer, since Dynaudio is well known for its studio monitors and audiophile speakers. The only downside is it being a passive monitor therefore, the quality of sound emanating from the
BM5 is directly dependent on the amp driving it. I suggest testing your amp on the BM5 if you could manage it. Just make sure your amp's power output is greater than the input requirements of the BM5.
There are other monitors in the Behringer's price range like the
Event TR5 and
M-Audio BX5 that have gotten good reviews in several recording and pro audio mags that I have read.
Take along some fave cds and try to audition monitors before you buy. Let your ears guide you when making your decision. Get the ones you feel most comfortable with because if you've ever mixed with NS-10s like I have, then you'll know what I mean.