If both posts had the same cross section, the titanium post would be stiffer since Ti has a higher modulus. However, since titanium has a higher modulus, is stronger, and is also less likely to suffer from fatigue failures if kept below its yield point, a Ti post can be made with a much thinner wall than alu, and usually is. If enough post is exposed that means that a titanium post can flex more without any significant risk of failure, and my experience has been that a 27.2 American Classic titanium post flexes more and is noticeably more comfortable than an equivalent alu post - or most carbon posts, the Ergon/Canyon VCLS post being a likely exception. Since alu has a fatigue limit and titanium, steel (and carbon?) don't, the chances of designing and manufacturing a reliable structure that flexes significantly are better than with alu, if my understanding is correct.
However, my observations are based more on reading, test riding and practical use, not engineering theory; I'm not qualified to debate that, and am not going to do so.
Later,
Stephen