Honestly don't know, we haven't seen either of them fight in a while so it's a bit silly to try and call. If I were going to base it on history, David Tua would be the safe bet - he is more experienced with the "high end" boxers and has a ridiculous amount of power.
In his career he had what seemed to be "lazier" periods where he entered fights in not the best form, and still managed to knock people out. Other times he had clearly trained very seriously and in many cases it really showed.
People seem to have thought much less of him after his Lennox Lewis round, as though because he as outclassed (which he was) meant he wasn't a serious fighter. Keep it in perspective, Tua wasn't able to beat one of the best full stop but stayed for all rounds and made the man work hard - was obviously not ready for it but anyone who thinks he wouldn't have learnt from that experience may be a bit naive. This time Tua has clearly taken it very seriously and physically seems to be in damn near the best form of his career, and has a LOT of experience and a crapload of power on his side.
The last public bout he had was about 2 years ago, was an international bout and he knocked the guy out in good tidy form - so back then he obviously wasn't struggling.
Here is a clip of his last fight...
http://www.youtube.com/watch?v=ZFyltFAz2OE
Toms - if Lennox Lewis couldn't finish Tua off, why do you think Shane Cameron can?