Well, tethering isn't normally a limitation of the phone, but rather the carrier. There are apps that allow tethering on Android devices, but I believe they have to be rooted. Right now I don't know of any phones that have native tethering abilities, but that doesn't mean it can't do it.
Keep in mind the Nexus One doesn't do tethering with out being rooted and installing a hack. If you're stuck between the two phones, the easiest way to decide is... 1. Think about your carrier. Do you have t-mobile? If so get the nexus one. Because it is likely you won't see the X10 with tmobile 3G capability for some time. If you have at&t, wait for the X10a to be released. 2. Hardware 3. styling.
If you're OK switching network for the phone you want then I guess it doesn't matter. Just remember, depending on which way you go early on you could be stuck with EDGE speeds only (X10 on tmobile, or Nexus One on at&t)