My friend and I are planning a trip to Japan, since we want to go to Kyoto, Osaka is an easy choice for a big city visit. I have been to Tokyo but my friend hasn't. Would it be worth the trip to go to Tokyo, or would we still get a good flavor of a big metropolis by just staying in Osaka?

I wrote a long reply to this but decided to keep it simple. Tokyo is the largest metropolitan area in the world at 35 million. Tokyo city at 8 million people is bigger than Yokohama, Osaka, and Kyoto combined.

You can certainly get a flavor of big city Japan in Osaka. Tokyo, tho, is on a whole different level.

The first quesiton would be how many days you're planning to spend in Japan. If less than 5 whole days, I would say skip Tokyo and concentrate on Kyoto with a day trip to Osaka.
I've been to Osaka on business several times, but really cannot remember any memorable sites/experiences. If more than 5 days and/or you can do an open-jaw (fly into one, fly out of the other), I think it is definitely worth going to Tokyo. The city changes constantly, and there are always somthing new you can discover.

I was never a fan of Osaka until my last trip, when we ate dinner at Ahn restaurant and spent an evening at SPA World. I would go back just to revisit both of these!

I agree that the length of your trip should be a factor. Kyoto can keep you busy for a while, plus you can do day trips to Himeji Castle, Osaka, Nara, etc.

You've been to Tokyo already. Did you love it so much you want to go back? If not, I'd skip it and let your friend see it on another trip. Osaka is a very big city, and while not Tokyo, certainly has the flavor of a big metropolis.

Thats a good point. I really want to avoid Tokyo if possible. I am using FF miles so I am piecing this together. I am trying to combing Hong Kong or Singapore into this.

in which case, it does make sense to skip Tokyo all together. You and your friend definitely would get a taste for an Asian metropolis if you're going to HK or S'pore.

JAL/ANA (Star Alliance)/CX (One world) fly between KIX and HKG. SQ (Star)/JAL/ANA fly between KIX and SIN. Better flight frequencies on SQ and CX respectively.
Though you may want to take note that these destinations are not really that short-haul, KIX-HKG is about 3-3.5 hrs flight, KIX-SIN is 6hrs.