HTC as you mentioned gives you Flash. That means when you're browsing the web on your mobile and go to a site that uses Flash for it's videos or part of how the site works, you'll actually be able to use it, whereas you're out of luck on an iPhone.
I have a HTC Desire (non HD) and found the battery goes quickly too, but there are ways to mitigate this. You can put a widget onto your phone "desktop" to turn off mobile data usage (internet over your phone network) when you're not using it. This has increased my battery life massively from less than 24 hours to closer to around 80hrs of medium use. Your experience with this will vary as you may use the phone differently.
iPhone, whilst having a smaller screen has a higher pixel density. The retina display is currently a class leader in that department. I.e. you can really really really zoom on text and the like without a loss of quality and pictures look excellent. Still, Desire HD has a super LCD screen which a lot of reviewers have praised as being excellent quality. Both handsets are reported to have excellent build quality. Apple have more apps for the iPhone, but are also more restrictive about what and how apps can be used.
Best thing to do is go into a store and play with demo models of both and see which you like best. A Desire HD will more than likely run you less money per month by about £5 - £10, which over 18-24 months is a fair bit (unless you plan to buy sim free).