Due an upgrade and was considering getting either a Nokia N8 or HTC Desire HD.
Any suggestions, advice, pros and cons of each etc

I'm a huge fan of Android powered devices so I would personally go with the Desire over the N8. There are damn near limitless possibilities with the Desire. I have a Droid (first generation) which is also Android powered. I will never get anything other than an Android phone going forward, unless of course I have no option and Android phones have vanished.

Is there really a choice here ;)
Froyo all the way...

Here's a nice side by side comparison for you: Compare Nokia N8 and HTC Desire mobile phones (http://www.esato.com/phones/compare.php?phone=620&cp=604) more here, but it includes the iPhone: http://crave.cnet.co.uk/mobiles/htc-desire-hd-vs-iphone-4-vs-nokia-n8-50000778/

I'm normally a Nokia guy but the symbian OS on the Nokia doesn't compare to the droid.