I'm still not able to get past the idea that once you have IOs slotted, you'll get better performance out of a Blaster. Softcapping Defense to Ranged attacks is pretty easy with use of IOs. And, really, that's what my prospective hinges on, the idea that ultimately, IOs will be used.
Of course, this also depends on how you're playing the game. If you're soloing, then Sentinel is absolutely the way to go. Blasters can slot into defense, but you'd probably end up with more reliability with not as much effort through a Sentinel.
However, if you're teaming, I think the Blaster has the clear edge. You're not going to get the bang for your buck from the defense secondaries, if you have a half decent tank soaking up the hits. If you're a Blaster, and you're in the fray, you're either a Blaster, or you're standing too close.
There's probably a few other ways you could slice this to show how one could have the edge over the other. Personally, if I'm playing in PvE, and I need an entire secondary dedicated to defenses, I'd be inclined to just roll a Scrapper or a Brute.
This is all speaking mechanically, from a min/max prospective, of course. There is no substitution for concept builds.