I'll tell you for sure - SiRF III is superior and just plain amazing. No doubt about that. I'm using mapless GPS navigation units in tandem with PC and PDA mapping software primarily in the car for the past 7 years. Quite often we're traveling to the same camping/lodging places by the same routes. So I know exactly the dead spots on these routes where my old GPS (simple yellow eTrex) can't catch enough satellites to pinpoint my position. I have them marked on the map. I've found the best place in the car for my old GPS unit, where it's working best (under the sunroof). And almost bought an external amplified repeater antenna for it...

About 18 month ago I've got the SiRF III based GPS unit. And started using it in parallel with the old one. 10 of 12 dead spots just disappeared from my map. And I'm not anymore concerned about the new unit position in the car. It works fine even in the glovebox! Usually it's just sitting in my shirt pocket all the time (it's tiny and Bluetooth connected). Our most visited urban maze is San-Francisco. My old GPS was not reliable at all near the downtown. The new one works just fine!

And that's just simplistic Holux GPSlim-240.