if you go for externals you wont get condensation and can have ther windows open a little, i went through all of this my self and settled for externals from silver screens. Not cheap but i think they will be worth it.
We have a full set of NWCC which we use for winter and find them excellent with the only draw back being condensation (the same with most internal covers so just give the windows a wipe as part of the morning routine and also gets rid of the sucker marks!)
Also have a Silver Screens external for privacy / black out (only side / back curtains fitted) in the better weather and, again, find this an excellent piece of kit especially with the "Priva-See" perforated panel which allows the front section to be folded down and still lets you people watch!
Because of the size, we only carry one or the other but seem to have judged everything OK so far!
ISOLITE window lining for the front windscreen and cabin windows provides insulation in winter and heat reflection in summer. Suitable for all Transporter based models from 2010 onwards and those with lining prior to this.