In reply to Rigid Raider:
> Good advice but SPD shoes, cleats and pedals are approximately double the weight of something like a Look Keo pedal, cleat and shoe
Sorry, but this is not true - the weight depends entirely on the gear you have, not necessarily the system you use. For example, Shimano A520 SPD road pedals are 40g lighter per pedal than Look Keo classics; both pedals cost about the same. My Specialized XC MTB shoes are the same weight as my Decathlon 3-bolt road shoes, and just as stiff (although about 30 quid more expensive).
What is certain is that there is a greater selection of lighter weight 3-bolt pedals and shoes, so if you wanted to spend more money, you can achieve lighter weight. But even if you spend 500 quid you wouldn't get anywhere near to halving the weight of an A520 pedal, cleats and half decent shoes.
FWIW I own both 2-bolt and 3-bolt systems, ridden both quite a lot, and prefer the 2 bolt system. Although I acknowledge that by spending more money I could get a 3 bolt system which is lighter and stiffer, but we're into marginal gains territory here.