Lump charcoal is 100% natural and pure, with no additives. It’s just wood, that has been burnt down until it’s pure carbon in an oxygen-poor environment at very high temperatures. It’s burn behaviour will depend on what wood it was originally, for example maple or oak give a steady heat, some other woods can be a bit sparky. Lump coal will burn hotter, but quicker than briquettes, so the burn time is shorter. Make sure you get lump charcoal that has been double sieved, so you're not paying for something that’s messy and you can't burn.
Briquettes have a pillow shape and are a composite fuel. They're a mixture of coal dust, accelerants, charcoal fines, binders and an oxidising agent. The binder makes sure the briquettes stay in their unique shape (it is usually a food grade starch), it also makes sure the briquette doesn't crumble. The oxidising agent ensures the briquette is easy to light. The rest is generally coal or charcoal fines that is carbonised hardwood. Some briquettes do give off a acrid smelling smoke to start with, so using a chimney starter is recommended. We recommend using Heatbeads which are odourless and smokeless. They provide high and consistent cooking temperatures.
We recommend you try both and see what your personal preference is. Many Pitmasters are polarised over this!