First off, changing probably wont fix the pixelation issues, that's far more likely to be a signal problem. The hardware is the same, so I would expect picture quality to be largely identical.
Secondly, it's not an upgrade, it's a migration to a different system (so like Windows to MacOS, or Android to iOS), VM are switching because the software on the 360 is owned by their parent company, whereas the TiVo software on the V6 is licensed.
The 360 is missing a number of features compared to the V6, a number of which are TiVo patents, so will probably never show (and TBH I woudn't hold by breath for the non-patented ones as it still doesn't have un-delete after years of people asking for it).
Some of the missing features are as follows; Quickplay (play 10% faster with audio), Wishlists (where you can set the box to record programs on certain subjects or featuring certain people), undelete, series link+, suggestions, setting of series links for programs not in the current TV guide, ethernet control, keyboard support via USB, turning off the mini-preview screen while browsing menus, switching between tuners with constant buffering, skip-back on fast-forward, default recording options, cancelling individual recordings from a series link, multi-channel series links, radio channels allowing recording of radio, separation of recordings between boxes, watching recordings when internet is not available and probably a few others I've forgotten.
Also, extra boxes do not have their own hard disks or tuners (unless converted from a V6 box), so you lose storage space and recording ability and are also reliant on the main box being on to watch and set recordings and also on having a decent wi-fi signal (or ethernet cable) between the two boxes when watching recordings. If the boxes are used by independent family members i.e. older children living at home, then there is also the issue of recordings being pooled across all boxes, so everyone's recordings are mixed up, rather than being separated. This also means that if there is a show that both people watch and one person watches and deletes it then it is gone for the other person as well, and can't be recovered due to the lack of un-delete.
