Gnome-terminal:
/bin/bash -c "TERM=screen-256color-bce tmux attach -t main || TERM=screen-256color-bce tmux new -s main;"
iTerm2:
/bin/bash -c "TERM=screen-256color-bce /usr/local/bin/tmux attach -t main || TERM=screen-256color-bce /usr/local/bin/tmux new -s main;"
As of 9/15/2016, my iterm uses bash -l tmux attach -t base || tmux new -s base
.