It really depends on how good a starter you make. When I make a starter, it's at least a 2 liter volume on a stirplate. This produces much more yeast than what is contained in two smackpacks or vials.
On the other hand, many folks make a starter with only about 2 cups of starter wort without a stirplate and aerate it by just giving it a good shake. This volume will produce very little yeast growth and won't do much more than wake up the yeast. In this case, you would probably be better off pitching two smack packs or vials.
If you are talking about dry yeast, you don't need to worry about making starters at all. Actually, making a starter with dry yeast will be worse than no starter or rehydration. There is also enough yeast in a dry packet, that it will seldom be necessary to pitch two packages.
Basically, to decide if two packages without a starter are more effective than one package in a starter, you need to look at your starter making process.
You should probably check out
www.mrmalty.com for better info on yeast and starters.
Wayne
Bugeater Brewing Company
