I got both.
The Nexus One has a classier, sexier body build and quality. The Samsung Galaxy S feels plasticky, but its good, solid, hard plastic that you feel confident will take a fall. The Galaxy S can be tacky---I don't know why for some reason, Korean phones love to use neon colors here and there. The Galaxy S has a lot of unashamed iPhone references, from the green battery view when the phone charges to the Touchwiz side swipe of the apps screen, which is unlike Android's vertical scroll. The Nexus One also has this funky glowing trackball that can pulse with different colors with the right app.
The Galaxy S has its own perks though:
1.85gb of internal memory. Yeah, you can store plenty of apps without worrying unlike the Nexus One with its 185mb. That's 10x more. I feel this is a very important advantage after the screen itself.
16gb built in SD, plus extra SD slot. The Nexus One comes with a 4gb card.
Has a bunch of nice widgets, such as one that is almost like HTC's Friend Stream on the Sense. Daily Briefings is also nice.
Comes with a bunch of nice apps, including Swype and Aldiko E Reader (international version only)
Got the best lock screen in in any Android
Nice improvements on the Notification Bar, which includes Wifi, Bluetooth and volume controls.
Not only does it has Android Market, but it has Samsung Apps, which is for Samsung's own Android apps and those they licensed. I can get Vlingo for free here. Its $10 bucks on the Market. You can find some games here. RoadSMS is an interesting augmented reality app. Samsung also has their own version of an Android Facebook app which does something the official Facebook for Android app doesn't --- record and upload videos on app.
Forward facing VGA camera (International versions only)
As a note, based on my exploration and ownership of the device, Samsung's own UI improvements are only skin deep. Compared to HTC Sense, Samsung leaves a lot of things stock, such as Gallery, Contacts, Calling, etc,. Given this, I feel its easier and faster for Samsung to adapt their latest phones to future Android releases.
In my usage, I find the Samsung Galaxy S to be liquid smooth but it would lag a bit if there is too much apps around, which is why I recommend using Advanced Task Manager to occasionally sweep and kill the open apps on the background.