Price is fixed and theres issues with some of those as well.
This quote from sat-shop.co.uk
"If you're going for HD and Linux then would be the VU+ DUO for me without question.... I personally would avoid the DM500HD because there is STILL this problem with them not booting up (received one back from a customer just this morning), the DM800 is actually discontinued and will be replaced by the DM800-SE later in the year, the DM8000 is far to expensive, the VU+ SOLO while working fine (got one running here great) perhaps doesn't enjoy quite the support you'd expect just yet (it will come), so for me leaves the VU+ DUO, great support, twin tuner, great build quality, the most popular receiver in Europe right now, say no more!"
DM500HD - No CAM slots, single tuner, single card slot, single scart, no RF out and uses eSATA drive for PVR plus other fuctions, no front panel display. Fairly easy to flash new firmware but requires you do this via USB lead connected to PC or via web interface (either way a PC is required). Runs open source Enigma2 (Linux) firmware and is supported by various third party groups (quite important when you are talking Linux and Enigma2). Cost £319?