theres more ram on the desire,
btw can you really get sense ui if you root it?
Yes there is more RAM on the desire, but less is available to the user until some modders "refine" the ROM. Sense UI also uses ~50MB of RAM in itself, so you're not really gaining anything in this respect. [When running the Desire ROM with no apps open I have ~180MB of free RAM, I had almost 280MB on Cyanogens Nexus ROM]
Yes you can only get Sense UI if you root, this is because it isn't an official ROM, and as such the phone won't let you install it if you don't have root. The Sense UI is a Desire ROM, which has been hacked to work on the Nexus - they are IDENTICAL hardware wise minus the noise cancellation and trackball/trackpad - both are made by HTC
Everything above is true, the Desire has a trackpad as apposed to the Nexus's trackball. Sense has nice widgets, but an awful launcher [See if you can get hold of an HTC Hero to see what I mean] but this custom skin means it will be slower to be upgraded to newer versions of Android - The Hero is stuck at 1.5
for now....
I've got a Nexus, and paid £410 all in for it, however some have been charged as much as £450 due to currency conversions and bank charges. This is split as ~£350 for the phone and delivery, paid to Google + £60 VAT which DHL pay for you, and they invoice you ~3 weeks later.
If that sounds expensive go and have a look at what the Desire is up for pre-order for on Amazon - £530 last time I checked. Desire is due for release on the 1st of April, the Nexus can be here in 2 days from the states