AVCHD or Blu-Ray

My camcorder a Sony HDR-CX 550 XR550, can record using different HD settings including AVCHD & for Blu-Ray. As I understand it booth of these settings are very good & that Blu-Ray is slightly better because of a higher bit rate used when recording.

Is there much difference in the files sizes & should one be able to actually see better results from Blu-Ray? If you only used stereo sound what would be the maximum running time that you could put onto a 25GB disc if your menu wasn't very large?

If after making my first Blu-Ray file I decide that AVCHD is good enough for my usage (I have a media player that can handle it) would I have to re render to AVCHD from my original files or would just converting the Blu-Ray file produce similar results?

Any other tips would be appreciated.

Thanks in advance