I was planning on going for the plastic jobby as looking like you at the photos it looks the lowest.Which one do you mean, Plastic Jobby, Classic (plastic) or steel?
Don't have a Mantec snorkle myself but looking at photos I have found on-line (and it is actually quite difficult to find photos showing the side elevation without looking from slightly below) I would say that the Classic (plastic) & steel are almost certainly taller than the roof but the Plastic Jobby looks like it is either the same height or even (very) slightly lower.