For the Desire HD they're all much of a muchness: you get the phone free if you pay from £35 or £40 per month on 18- or 24-month contracts. You can find deals for as low as £15 a month, but that would mean a typical £250-plus upfront.
With the Qwerty-sporting HTC Desire Z, deals are only marginally cheaper, even though it's a less powerful phone. Again, you can get the Z from free on £35 to £40 per month contracts, and £15 to £20 contracts are also available if you want to pay a thumping great wodge of cash up front, albeit generally less than the HD.
Importantly, these are phone plus SIM deals from various independent
retailers -- they're buying the phone and adding a contract SIM from a
network. They're not direct from the network themselves, so you won't be
able to upgrade this way, at least until the networks announce
As for SIM-free versions of the phones, the Desire HD is going for £500, while the Z is going for £469, but as we've reported previously, you can find them cheaper. They'll be dispatched by 6 October, according to Mobile Phones Direct.
Comparing to other phones isn't an exact science, but at first glance, the HTC Desire HD deals are cheaper than what the BlackBerry Torch and iPhone 4 are being offered for, and on a par with deals for the Nokia N8, on both pricing and the range of networks offering it. Sound about right to you?
Update: A previous version of this article didn't make it clear that the deals on offer are from various online retailers and not from the networks themselves, so won't be compatible with existing contracts for your upgrades.