Sky HiDef package will require you to have a display that is capable of handling HDCP inputs (via HDMI only at present) as they can't use VideoCrypt on the HD transmissions.
There's rumour that their Hi Def Sky + box will start appearing very soon with HDMI outputs
Btw over Sky+ TiVo give you quite a bit...
Full and frank comparison of the two here
http://www.tivoportal.co.uk/stb.htm