well it all depends how old you are, i've been playing tekken since it first came out so there for i am more likely to buy tekken. soul calibur is a pretty cool game mainly because of the newer characters involved in it. but they mainly are the same game. but both games are basically the exact same type of game so its hard to say which one is better than the other.
Tekken has the better fighting system of the 2 but soul caliburs ability to negate pop ups by controlling where you fall is an added bonus especially if you get caught up in the combo mania. Every character in Tekken has his/her own special style and the same can be said for Soul Calibur (minus Seoung and Kilik)...actually you know what..Tekkens better lol
Tough choice really, Ive played both and I have spent alot more time with Soul Calibur rather than Tekken but I did enjoy it too. Fighting mechanic wise, Tekken is the better fighting game here, there is alot of imbalance in Soul Calibur. SC is like MK and Tekken is like SF, just 3D arena style.
They're both great series of fighting games. I feel Tekken is more for those who like to get really hardcore in their fighters. Memorizing a bunch of combos and practicing to prefect technique. Whereas Soul Calibur is a little more friendly to more players. Bit of a button masher, but if you take the time to learn some combos you can be almost unstoppable. Plus fighting with weapons is pretty awesome. I'm leaning more towards Soul Calibur.