They will each add a slightly different flavor profile. I like using White Labs WLP002 English Ale yeast and WLP013 London Ale yeast for bitters. IMHO, the WLP013 will be a bit earthier and profile the hops; while the WLP002 will be clear and a bit sweeter. One of my favorites is their WLP007 Dry English Ale yeast!
I've yet to do it, but I know that some people will split a batch into two separate fermentors and use two different yeasts so that they can taste the difference.