Venetian rooms are far larger and nicer than "normal" Bellagio Rooms.
Bellagio rooms with view of lake are nice since you can watch the fountains and
tune your TV on to hear syncronized music - but they normally charge something
like $50 extra a night for the view (not worth it). You can try to get a
host to book it - but sometimes you won't get it even if they request the view.
Restaurants in Bellagio are better than similar hi-end restaurants in
Venetian in my opinion.
B has a lot more $1 9/6 JB than V.
I believe you will need to play more at B to get same offers as V. Also
Venetian has more slot tournaments (and thus tournament offers). Venetian also
has Gold program which gives you higher cashback if you play a certain amount
to get on their Gold or Preferred player list. Also this gives you access
to the Gold lounge with free soda and limited snacks. (You can usually walk
in without having a Gold card). Foodies at B can probably snag a little in
free snacks if you wander into high roller lounge too...but at V it is much
easier to do this.
$1 and up players may be able to get comps at B for Cirque du Soliel shows
there and at other Mirage properties. V currently has no shows.
Overall - for room quality I would stay at V. But you can visit B too and
play a little and maybe get some offers there.
[Non-text portions of this message have been removed]