I'm not an expert but just someone who has used TiVo from the beginning. I think you maybe looking at this backwards. Work out what you love about TiVo & the features you use the most then compare those with the Sony. You will probably find that whilst the Sony covers the basics, it will not deliver the sometimes intangible 'TiVo' experience. SKY+ delivers all the basics but the user experience is awful.
That said, I think if I was using an unmodified TiVo I could be swayed by other technology, but using a networked, fully hacked up TiVo the experience is almost second to none.
I am playing around with an HTPC front end called xLobby & it looks very promising, but at the end of the day it is still a PC & not a plug & play (turn on & leave) device.
Finally, there is probably nothing stopping you using them together?!
Martin