Tiggs, sorry a couple of things: toxicity, neither are toxic. They both are narcotics in and asphyxiants though. Butand is heavier than propane nope, molecular mass is higher but one kg of each weighs 1kg densities do differ but not by much.
Heats of combustion -2220kj per mol, v's -2877 kj per mol, so more energy for a given amount of butane than propane.
Balanced equations show that for complete combustion you need more oxygen for the butan therefore more likely to get CO from the butane for a given amount of air.
To answer the OPs question no you won't notice much difference unless you are at low temperatures in which case the bp of propane is more suited to your needs but for UK/euopean camping over the next few months either will do.