But because titanium is such a poor heat conductor compared to aluminium don't you tend to use and therefore carry more fuel, thus losing any weight gain from the cookware ?

While that's certainly true, titanium pots are generally thinner than aluminium pots. On the whole, I don't think there's a significant difference.

That said, I use an aluminium pot because it's light enough with excellent dimensions and I haven't come across a titanium pot good enough to justify it's price (here in Finland there's not quite as much to choose from and ordering from the net would be even more expensive).