A good source of reviews is here
http://www.camcorderinfo.com/ratings.php
You have to translate in some cases to the US model numbers (the SR100 is like the SR90).
The review of the MG505 is more recent than the SR100, so it makes comparisons to it.
The SR1 does give you the choice of recording in SD MPEG2 format, or HD AVCHD/MPEG4 format. I'm not sure how the SD compares to the SR90 or the JVC. I would expect it to be similar - as good, but I don't think significantly better.
Generally speaking larger CCD or CMOS is better in low light; 3 CCDs can be better in colour accuracy than 1.. but that isn't always the case. Also CMOS has some advantages over CCD. I suggest not getting too bogged down in this and look at the overall performance of the different models, as it is all the parts together (sensor, lens, DSP, etc) that make the whole.
I have a tape based HDV camcorder, and whilst it can shoot either SD or HD, I always shoot in HD, and when making SD DVDs downconvert in the software; the quality looks best to me that way. The downconversion in software is fairly easy for HDV, but currently this is not the case for AVCHD (there is no software support for it yet!). The SR1 comes with some software which does allow you to convert to MPEG2.