After a bit of reading up during the build of my project bike, I chose to got the lithium route mainly for the weight saving - its gotta be the cheapest way to save in excess of 3Kg!. During its as yet short service of around 1200m it has worked brilliantly. After assembling the project build I connected up the lead acid battery from the road bike to check over the electrics - but it struggled to churn the engine over on prodding the start button (1225cc, High compression motor). Swapping to the lithium battery, the engine spun really well and fired up the fresh motor no problem, so LOTS of cranking torque. From then, its been Easy Starts, and no issues at all. I also fitted a lithium friendly reg/rect and bought a lithium trickle charger for battery care over the winter. The battery is an AntiGravity AG1201, and the reg/rect is from Ricks Motorsport; 14-001H. Received wisdom says replace the reg/rect as the stock one for lead acid can fry the battery with voltages over 14.2. The pairing works well and maintains 13.5v while in regular use. The bike itself is not at all stock and has a full Motogadget electrical system, which also works well but is a PITA to install!
Overall, I'd recommend the change to Lithium if you are on a weight saving mission, but the stock lead acid has always worked well on my less modified road bike and I wouldn't spend the money just for the sake of it -£582 for battery, reg/rect and charger!!