I own both and let me tell you this.
I played Tekken 5 for 2 weeks tops. The only thing worth doing is beating the arcade with every character. The Devil Jin minigame is crap. Also, a lot of the characters seem like very close duplicates to each other. I haven't played it since I got it, which was months ago.
I got Soul Calibur 3 almost exactly 2 months ago and I am still playing it today. It's a little more challenging, which is better imo because a challenge keeps you coming back for more. There are more characters, and each character has a unique weapon, so the characters are a lot more diverse. You can beat arcade mode with all characters, and even doing once isn't enough, as each character can take seperate paths, providing for different cutscenes. There is also a load of unique minigames and Chronicles of the Sword, which blends Soul Calibur and strategy into a whole new game. A lot of people thought the mode was not worth it, but that is why they put it in extra. Personally, I love it. Also, you gain money in pretty much everything you do, which allows you to buy more weapons and parts for the normal characters, or custom characters of your own. Which leads me to Character Creation. You can make your own character, choose their gender, their occupation, their weapon, and then their appearance, from the socks on their feet to the hair on their head. It is very diverse and chock full of replay value, providing for a lot more fun imo.
Definitely get Soul Calibur 3.