Originally Posted by big_ajk
What is the difference in the final product though? Are there different flavors, more or less bitter, etc.? it would be neat to see if there was a big difference in the types of yeast used.
Yeast can make a HUGE difference in your beer, but generally with a porter, you should be using strains of yeast that don't impart much flavor.
For the three I posted, here are the differences (all fairly subtle):
Nottingham will finish fairly dry with little to no discernible yeast flavor or nose. It is very aggressive and will likely have the shortest fermentation period of the strains mentioned here. Because of its high activity, it can also create substantial heat during fermentation, so you'll want to keep the fermenter in a cool place - 60-65 degrees.
US-56 is an American ale yeast that ferments a bit less aggressively than Nottingham. It will still leave a very clean flavor profile and will likely not be quite as dry as a beer fermented with Nottingham.
S-04 is an English ale yeast that ferments fast and flocculates very well. It will likely produce a very clear beer relatively quickly. S-04 will finish sweetest of the strains I mentioned here and may produce a few esters, providing a very slight fruitiness that isn't necessarily objectionable. The warmer the ferment, the more pronounced the ester profile.