Does your camera record the Audio as DD 5.1 SS? If so, that will determine which version of the AVCHD Preset you will choose. You will have little issues, if you do not accommodate that Audio format. In your case, you can let PrE do the down-sampling to SD.
Also, does your camera allow you to set 4:3, in lieu of 16:9? Does it allow you to record stereo (2-channel), in lieu of DD 5.1 SS? Does it allow for down-sampling in the camera? The more that you can do in-camera, the better off you will be.
As an aside, I would not use DD 5.1 SS in camera, if it's possible to choose 2-channel. DD 5.1 SS from the camera is a gimmick in my book, and causes more problems, later on. To record multi-channel Audio, that is clean, is a big undertaking, requiring 6 specialized mics, a mixing console and a multi-channel recorder. It is best done by a skilled sound crew. I have never heard on-camera 5.1 that was at all useful. Either 1-channel (Mono), or 2-channel (Stereo, or dual-Mono) would be better choices. Usually, one wants to isolate the Audio, and NOT include extraneous signals. That is how the pros do it.
Let us know about your camera, and which settings you can use with it.
My camera is Sony HDR-XR500V - I recorded in the HD default setting with
only the camera mic. I believe it recorded in 5:1 because I didn't change
the selection. I do have the option to change to 2 channel but I didn't
know that when I started! I cannot find any way to down sample in the
camera. I cannot find any way to down sample in the camera.