Nice comparison of the three USB GPS receivers! This was done indoor, in your house, wasn't it? A few years back, with older GPS technology, that might have not been possible. GPS receivers were less sensitive, and you would have difficulty getting any 3D fix.
In my experience the new u-Blox 5-based unit is not as sensitive as the other two, just like you said. It's likely due to the smaller size of the internal antenna. Yet, I would buy it over the GPS-500 unit just because of the issues with the bad mechanical design of the adapter/connector/USB plug. The new Microsoft unit has been designed better, and I don't think we will be hearing from users who damaged it, unlike the old unit.
I've been using the new
Microsoft USB GPS "Locator", Navation 168, with u-Blox 5 chip, for several weeks weeks now, since my BU-353 failed, and it works just fine. It sits on my dash board, and I use a USB extension between it and my laptop. The only thing to add is that the Microsoft logo must face up, towards the sky, for it to perform better.