Each solution has its merits,
internal silver screens (I have) have been great, used on T5, 5.1 & T6, simply bung them up, crack the side windows to create some air flow and prevent condensation (wind deflectors to drivers and passengers windows ) black out is pretty good but not perfect, (good enough) they reflect sunlight really well and keep the van really cool.
pack up small / flat and stay dry what ever Mother Nature throws at you.
can be folded back behind the door lock pop up button so you (others can see the alarm is on )
externals are truly black out if fitted correctly, more bulky and a pain if it rains and they can be wet when packing up and need to be dried to prevent potential mildew.
can cause additional noise in high winds, some have reported rubbing of the paint work (DYOR)
they reportedly Prevent potential condensation via insulating effect from out side.(although the occupants still breath out a couple of litres of water each, so still need to crack a window potentially) Cooling effect should be very good as insulating effect will be high.
externals can have funky designs such as eyes etc