PET seems to be more brittle than HDPE, so it might be more prone to break if you drop it. I use them in one gallon size which makes them pretty heavy when full.
I also have some HDPE bottles that have a nylon interior coating that is supposed to improve the barrier qualities, but I'm not sure exactly how that plays with photo chemicals. Nylon's permeability can decrease in water, or so I've read. On the other hand I have some Xtol stored in them that was mixed up in July of 2011 and it worked fine as of last weekend.