I looked at the Onix after purchasing the Garmin 400t. The Onix is a very sweet unit and offers quite a lot, but two major items lead me to stay with the Garmin: the Onix's lack of speed (slow graphic rendering and a frequently aggravating inability to properly refresh the screen.) Sometime half of the old map would jigsaw its way to the new view etc. Secondly, it is largely an untested group of technology. I am sure it works fine, but I want to be able to trust my equipment. Out in the field is no place to discover a glitch or bug in the system. Especially if the field is Alaska or Northern Canada.
The 400 is not without faults, but they are solely centered around an awful user interface. The unit grabs GPS sat. signals and wont let go - offering a very accurate position / routing capability. The maps on the 't' are top shelf and the screen is just beautiful to look at.
Good Luck - let us know what you decide and why!