Generally speaking, if you want light and narrow at the expense of comfort and ventilation, the internal- or frameless-backpacks and rucksacks are better.
If you want comfort, ventilation, high-placement of your load and/or the ability to carry more load, an external-framed pack will be better.
If you want what's most trendy, the last few decades give you an internal frame and these days you need some translucent, disco material to really make the grade. All the cool kids have it. The link to Roger's home-built, UL external-frame backpack with UL materials crosses all boundaries and must therefore somehow be immoral and likely illegal in Australia. :)
"Generally Speaking", implies the existence of exceptions and acknowledges that we might all be "more comfortable" with the product(s) we've invested in.
The largest-capacity packs one can buy are internal-framed rigs with far more capacity than any commercially-available, external-framed packbag. I think back to a couple we met on the PCT a few years ago, he with a giant, blue cordura internal-frame that must have been 40" tall and which he reported to be 80lb for their 7-day trek. when he set it down, I think I felt it thump the earth... His comment at my Kelty was that he used to have one, but it couldn't carry enough stuff! All I could think was how that entire load rode right on his back. Nuts, man. Probably very comfortable!
Noting the absence of UL-Externals on the commercial market, it's also notable that the typical internal-framed backpack, say at REI, weighs far more than an external-framed pack of similar volume capacity. They're heavier, less comfortable, more expensive and cantilever the load out off the back with a narrow profile. This is another reason why the reliable, light, comfortable, ventilated external-frame packs are still preferred by many youth and adults that care (and that carry higher-than UL load weights). Go with the stuff that weighs less and works better. How cool would it be to have external-framed, commercial packs with UL-quality packbags? VERY.