Most folks that ever cooked with lump charcoal such as Royal Oak, Wicked Good, Big Green Egg, or even Cowboy have likely noticed that the amount of ash produced from lump charcoal versus Kingsford charcoal briquettes is miniscule.
My seat of the pants research tells me that although charcoal briquettes burn longer, food cooked with lump charcoal simply tastes better. Charcoal briquettes contain various types of "fillers" that are not naturally found in wood.
But, don't take my word for it. There's a web site that provides all the analysis and insight into the advantages and disadvantages of lump charcoal versus briquettes. Visit NakedWhiz.com to see for yourself. Or, better yet, conduct you own research using the methods employed on NakedWhiz.com using charcoal that's available where you are located. I've never tested it, but I suspect that Royal Oak briquettes will outperform Kingsford in most of the parameters tested on NakedWhiz.com.
It sounds like I've found my next bbq project for the off season--charcoal testing.
If you've done your own testing and would like to share the results, send me an e-mail and I'll post your findings or just click the comments link to this post and publish them to the blog directly.