I'm in the fortunate position at the moment of having both those phones in my possession at the same time.
The Nexus One is a rare one....supporting the Telecom XT 3G frequencies of 850MHz and 2100MHz. It's stock and un-rooted, running the latest Froyo Android v2.2.1 "FRG83D". It updated, over the air, in my hand about 20 minutes after I turned it on. It will probably be the first phone to get Gingerbread (Android v2.3).
The Galaxy S is running Froyo JP6, rooted with z4root and using the (essential) One Click lag Fix (OCLF).
I'm going to sell one of them.....and I thought it was going to be the Nexus One.....but I've been using it the past 24 hours and it runs sooooo quickly and smoothly. In many ways that matter to me the Nexus one is the better phone.
The Samsung is very close once it has been "fixed". But I think that's exactly what's bothering me about it. Out of the box the Galaxy S is second best to a Nexus One running stock Android. Yes, the Galaxy S has more internal storage and a much more richly-featured camera. Both of these things matter to me a lot. It has a bigger, awesome screen. It's lighter, yet powerful. Froyo has been slow to come to the Galaxy S. Nexus One got it in May. Kiwi Galaxy S owners via Vodafone are still waiting. How long will we wait for Gingerbread? I hate Kies. HTC / Nexus One's OTA process is *much* better.
But the bottom line is the Galaxy S just doesn't run as smoothly as a Nexus One. I can feel it. This matters to me.
More later.