Dear Manuel,
I asume that you have a WM 5 running device.(probably MDA Pro
)
In this case i would suggest that you don't use programs that need a lot of memory while listening to music.
Second the quality of the sound via A2DP is better than with BlueMusic as A2DP uses a larger badwidth (so i higher bit-rate -> better quality of your mp3).
Still you can use BlueMusic for other things like Skype or other Voip software (that normaly uses the front speaker and mic), have your gps system indications directly to your headset, record notes.
You can always download the trial version and see if it suits you for any purpose.
As for music enjoy your stereo headset with A2DP, maybe check the t-mobile service site for newer RUUs.
Best regards,
Raul