Turbodiesel -thanks for the background on the Marathon range.
Ian, similar to you, weight is the big issue, so I definitely want to go for an airfloor. The ideal would be a Zodiac Futura FR, but they just seem too heavy for me to handle. I also want something at least 3.8m, so as you say, looking at Quicksilver, the 380HD is the only option, but far too heavy for me. Current choice for me seems to be the Zodiac Classic at around 50kg for the 3.8. The 4.1m is only about 5kg heavier (and about £200 more), so that seems quite attractive for a bit more space. However, other SIB's (like Quicksilver - and your Honda) seem much better value for money than the Zodiacs.
Is your Honda 20 a 4 stroke? I've pretty much discounted 4 strokes because of the weight (and cost). The Mariner 25HP 2 stroke is 51kg (just manageable), but the equivalent 4 stroke is 71kg, which is too much for me. I'm not too worried by noise or fuel consumption differences.