There are 2 variables in play. You can't look at them individually. I do know that absorb has more relative value for an Assassin thanks to Dark ward. But point for point, Defence should come out ahead assuming equal points in each. It's also worth noting that, since Defence is rolled first, higher Defence chance lowers the worth of Shield and Absorb,
First of all, ths is a no-snark reply and I'm not an expert (it's so hard to convey tone in textual format); I only know what I've read and even then, the finite mechanics are only truly understood by the dev's. End disclaimer.
Defense will be superrior percent for percent, not point for point. Defense, after somewhere between 25-27% has a stiff diminishing return. Absorb has a much lower return, which kicks in at a much higher percentage. Also, absorb gives more percentage gain per point of rating than defense. So you will never have a "point for point" exchange between the two. Past an undefined amount (at least, I can find no hard number), the amount of defense rating you have to stack to gain even 1% when compared to the amount of absorb (and thereby mitigated damage) you would gain by replacing this with aborsorption is unbalanced. At that time, it is then more beneficial to stack absorb than defense; the defense you would have to stack to get from ~27% to 30% could easily get you around %50 absorb. the additional 3/100 damage not taken, when you could mitigate 25/100 of all damage [(.50 shield)*(.50 absorb)] would be inferior.** Of course, this is predicated on taking shield/defense and shield/absorb mods, because as you know absorb does nothing without a high shield rating. The magic numbers that I see in most technical reviews of how the three mechanics synergize is D% ~27-30%, Shld% ~50%, abrb% ~50% (for guardian tanks anyway; different for Shadows and Sins b/c they have a much higher shield% potential). I write enough term papers every semester that I don't need anymore practice citing sources, but the info is outthere for anyone who wants to know bad enough to thoroughly research
Given the fact that all of us have different ideas or methods and we're all still out there tanking with varying degrees of success, I say "forgedda 'bout it" and go have fun!
**However, up to around ~27% defence chance, defense is superior because it gives comparable gains, and = a "no damage chance" vs a "reduced damage" chance.