I think tabbed windows take a smaller toll on the system than multiple windows. After all, Safari only has to load one window and all its attributes into memory with tabbed windows, whereas with multiple windows you could have several different window states to remember, sucking RAM like lemonade on a hot day.
Sometimes I use a mix, where I'll open a new window and open a couple related tabs within it, so I might have one window for email (and any sites I open from that), one window for forum browsing, etc.