I ran both setups in my house (WMC and Tivo) - I appreciate all the flexibility you get with a PC but hated the extender options, and WMC often had issues with updates etc.
A Tivo with a few Minis provides a much better unified experience IMO, especially when you get almost all the features of a PC-based system with add-ons like Plex, pyTivo, and kmttg. Plus all the net streaming apps that Tivo added that couldn't be done in a unified way on WMC (Amazon Prime, Youtube, Netflix, Hulu Plus etc.).
More importantly for me, the Tivo has a much higher WAF overall and includes Xfinity OnDemand which the wife uses all the time. The UI is easier to use and just works plus I don't have to boot up power-sucking Xboxes all the time.