One should always use liquid yeast over dry yeast.
(from Mr Malty
, Wyeast, White Labs, and several online supply stores)
A single 11g dry yeast packet, properly rehydrated, will result in a pitching rate of approximately 200 billion cells.
A single Wyeast Activator pack contains approximately 100 billion cells.
A single White Labs vial contains between 70 and 140 billion cells.
When making starters from vials or smack packs:
1 liter starter = about 150 billion cells
2 liter starter = about 200 billion cells
1 liter starter, then pitched into 4 liter starter = 400 billion cells
A five gallon batch of beer at an OG of 1.055 requires approximately 200 billion yeast cells for proper/healthy fermentation.
A properly prepared 2L liquid yeast starter is required to get the correct pitching rate for the aforementioned five gallon batch. Any error in procedure will decrease the cell count and/or introduce contaminants.
Many of the same strains are available in both dry and liquid form.
Dry yeast packets usually cost less than $2.
Liquid yeast vials/packs usually cost around $7, not including the cost of producing starter wort.
So, if the required strain is available in both dry and liquid form:
Choose dry yeast to minimize cost, minimize effort, and maximize pitching rate.