I have both, and it just depends on where I am going as to which one I will take. Down here in TX, it gets so friggin' hot that you will do anything for relief. An external is a blessing for its ventilation. I also like all the pockets, compartments, etc. on my old external to keep everything perfectly organized a lot. I carried it comfortably for about 10 days once in pretty different weather systems, so I had a ton of gear. Still, it was pretty comfy. It has seen plenty of 2-3 day trips where I am the pack mule for two when camping with the LOML, and it has been my pack of choice for that role.
To me, it seems quicker to unpack and pack up the externals as well. My external is a web of compression and attachment straps that connect to hard plackets sewn into the pack. The attachment straps don't ever come off, so all I do is roll up the bag and click it on the pack. Same with the tent. Once you get your arrangement down, it's almost modular.
As for the internals, I hate having soaking, sweaty shirt material scrubbing on my back with every step when I have my internal on. If the load shifts, the pack shifts, or a strap slips a bit, it seems like that sweaty bag of nylon mesh riding on your back is almost easy to leave behind.
That being said, since it is just a big bag, I like the convenience of throwing everything in it, and in moderate weather (just two short seasons here) it is great. Since I tend to keep things in little stuff sacks, compression bags, and little boxes, I don't need all the compartments. In cooler weather, I like the internal. It sucks in summer time S. Texas heat. If the weather is cooler and I am packing just for me, I like the internal.
I have a newer internal I am going to be trying out pretty quick, and it actually has lumbar pads for the sides of your back as well as an adjustable pad for the small of your back. It looks like it might do a good job of keeping the pack from laying like a sweaty bag of sand on my back. Guess I'll know soon enough!
Robert