I think it all depends where you get the phone from. Orange, Phones4U and most Orange stockists will sell you their crapware filled, branded, crippled Desire that really is an awful experience in comparison to stock Desire. Thankfully this branding is pretty easily removed.
The Carphone Warehouse will sell you an unlocked, branding free Desire on contract from Orange etc. and they may give you quite a good deal.
As for the Nexus, it has pros and cons. It has Froyo, gets updates seemingly quicker than other Android phones, and has a uni-body design (honestly, it's not that much different). The Desire IMHO is better due to HTC Sense, more RAM and physical buttons which are more responsive than the touch buttons on the Nexus One.
It's your choice but I'd go with the Desire -- preferably NOT on Orange (they even give you a measly 500mb data plan [I've had the phone three months and gone over twice due to Spotify and podcast streaming] unless you pay another tenner and their customer service sucks) or via The Carphone Warehouse. Look around for the best deals