I wasn't entirely sure what the SD90 was, but looking online, it seems to have much the same file format as the TM700 / SD600 range that we talk about here.
My workflow is as follows, but I'm sure there are many alternatives.
1) First I record in 1080p at the full 50fps - For best picture quality
2) iMovie can't at the moment see movie files on the camcorder recorded at 1080p50. This is because iMovie is designed to work with AVCHD, and 1080p50 is a later, more advanced video format that is subtly different. Different enough that iMovie won't see your movie files when you plug the camcorder in. When the disk is full, I record a tiny clip at 1080i which enables iMovie to 'see' the attached video camera, and use the Create Camera Archive option (can't remember exactly how that is phrased, but it creates a clone of your camcorder disk, so you always have the original version to go back to one day. You can also use Super Duper or even Disk Utility I think to make a copy of your original SD Card or Hard DIsk. This will be in original (best) quality and smallest file size.
3) Even if you don't create the archive (or if the disk isn't full yet) you can still plug your camcorder in at any time and see the contents via the Finder. I take it you know where the files are stored ?? The route is well documented here - Can't remember it off the top of my head but its buried deep in the folder structure in folders such as AVCHD, Private, BDMV - Look for those and you'll get to know them when you see them !! I drag the files as a batch out of that folder into a new folder on my desktop - This should copy rather than move the files. Once they are safely on your desktop folder, I drag and drop them onto Alan's automator script Rewrap2M4V which creates new M4V versions of each of the video files. It keeps the name the same and takes just a few seconds for each file to 'repackage' the movie into something the Mac can read and work with - An M4V file.
4) So, you'll now have a folder on your desktop with two lots of movie files, the original mts files, and the new repackaged M4V files, which are fractionally smaller than the originals. I throw the mts files away to free up space - I've always got the archive for later. The M4V files are high quality 1080 progressive video at the full 50 or 60 frames per second. You will need a fairly high spec Mac to play these at good quality, slower Macs will stutter.
5) What you do with these files is kind of up to you. I still like to use iMovie 06. This can't work in 50fps, but I like progressive, so I create a new 720p25 project, import the files and it converts them (over quite a long time) into very large, but stunningly good quality 720p25 AIC movie files to be edited. I've recently started migrating more to iMovie 11. For this, I tend to convert my files to AIC first with MPEG Streamclip. Give it the parameters to convert the M4V files to Quicktime AIC files at 1280x720 at either 25 or 50fps. My older Mac can handle these no problem, and iMovie 11 is the first incarnation to handle 50fps. I still find its not overly well supported, and has caused me a few issues so I tend to stick with 25fps, thereby reducing my resolution from 1080 to 720p and frame rate from 50 to 25, but they still look good. I have played with previous model iMacs using i5 processors and found these to handle 1080p50 pretty well, so I can only assume that these very latest Macs won't have any issues at all. Once I can prove that, I guess I will keep and edit my video files in their M4V second generation repackaged files, and avoid the whole step to AIC.
Hope this helps. Remember that you need Perian (a free System Control Panel) for Rewrap2M4V to work properly, and there are free utilities such as Movist, (my choice), for playing the original mts files without any conversion. But, you will need to convert to edit. One more thing, if you delete your hard disk or reformat your SD Card, the TM700 range will reset the counter and start recording video files starting with 00001.mts. I always found this a bit of a pain as I like to keep numbers unique and progressive (much like my footage) so I don't delete the whole card. I delete all but the last couple of files and it continues the numbering sequence. This does still throw up the occasional problem (ie when you delete files mid way through a card) but it is something to think about if you want to keep some order to your files