since these are consumer camcorders and
megapixel count is not a measure of video resolution or quality, the megapixel count is not too helpful. Video resolution is measured using horizontal line count. Video quality depends on the amount of compression used (less compression can improve video quality), lens diameter and imaging chip size. Other useful items are a mic jack for external mic connection and manual audio gain control. Megapixel count is useful to determine the still image resolution.
Both camcorders record the same high compression AVCHD format.
HDR-CX190: not available
Canon HF R306: not available
This means there are no threads for mounting filters or add-on lenses. Generally these entry level cams are in the 30mm diameter range. This means we can only hope the imaging chips is larger in one of them to determine which should have better low-light.
Imaging chip size:
HDR-CX190: single CMOS 1/5.8"
Canon HF R306: single CMOS 1/4.85"
Winner: Canon (larger is better)
Canon HF R306: No
manual audio control:
Canon HF R306: Yes. Limited to "Normal" and "attenuator on" for loud audio environments.
Canon HF R306: http://gdlp01.c-wss.com/gds/1/0300006931/02/hfr36-hfr37-hfr38-hfr306-im2-p-en.pdf
And we don't know where you are - so you understand that the Canon Legria is PAL localized - and not exactly made for use in NTSC countries... If NTSC use is appropriate, then the Canon Vixia should be investigated.
With the larger optical zoom range, bigger imaging chip and (limited) audio gain control, I'll go with the Canon.
I hope your computer has enough horsepower to deal with the AVCHD video and I hope you have an AVCHD-capable video editor. There is not much in the box with the camcorder that will he useful.