As with double glazing, it's the air gap that provides the insulation. TripleWall gives you 2 cavities, where TwinWall has only one.
In a domestic situation, it would be better in that respect, but I doubt if the tortoise would notice :o)
Either way, it's not a great product for insulation purposes, so I'd just go for the TwinWall.
There are various fixing systems for conservatories and greenhouses, but for a simple enclosure, Simply screw it on. Drill a 4mm hole first, then screw using a plastic or rubber washer under the screwhead. The DIY store will have suitable roofing washers. Maybe use those plastic ones with the little "flip-off" top-hats.
http://www.theplasticpeople.co.uk/twinwall-polycarbonate-for-cold-frames-clear-p-373.html?utm_campaign=google-product-search-us-en&utm_medium=product_search&utm_source=google-product-search-us-en&gclid=CICyq_vZuLgCFbIPtAodq3MA7Q