I woudl recommend transcoding to ProRes.
Keep in mind, thta you're going to need storgae for this. As ProRes, depending on which version you use will be 3-5X larger than your AVCHD files.
My prefernce is to transcode to ProResLT.
The reason being that the file sizes to image quality is very good.
Unfortuantely FCPX doesn't offer ProResLT as an transcode option on import, as they only offer ProResHQ, ProRes422, nd ProRes Proxy.
If FCPX offered transcode to ProResLT on import it would save me some time on the front end starting my project. But I don't need the size of ProRes422. So the extra ime is worth it.
Overall bty transcoding yoour fooatge to ProRes, you will save time and headaches. As AVCHD media doens't hold up well to compsitng or effects too well. No to mention you're most likely going to export to ProRes, so by having ProRes footage your encode will be MUCH faster, than native AVCHD.
Oh and most important, if yo wind up doing multicam work then ProRes will greatly help you out. As the high compression of AVCHD will be very taxing on your system with 3 or more clips.