The coolest feature of recent versions of iTerm is the "Selection respects soft boundaries" feature: with it on, iTerm will detect vim/tmux splits and constrain the selection to just one side of that split. For my workflow, at least, it makes a huge difference.
Also, if you start tmux as "tmux -CC" iTerm will open the tmux session in a new window, with GUI tabs for tmux tabs and GUI splits for tmux splits.