OK, here are the final numbers. To remind everybody, on February 18, I brewed an 11 gal batch of wheat beer. OG = 1.044. I had some old yeast slurry saved from a previous lager batch that was brewed on Thanksgiving (Wyeast 2035). I had periodically been "feeding" this yeast with shots of DME wort. On brewday, I took this old yeast and made a new starter from a 1/2 gal of my new wheat beer. I also did the same thing with a brand new smack pack of 2035. I let the starters do their thing over night while I let the remaining 10 gal of wort chill and settle in the fridge. The next morning, I racked the work of the trub into new carboys and pitched each starter into the wort at 50F.
Both took off pretty quickly. 7 days later on 2/25 the gravities were:
New yeast = 1.013
Old yeast = 1.014
Today (3/21/2011), after lagering at 33F since 2/25 the gravities are:
New yeast = 1.010-
Old yeast = 1.010
I put the minus sign after the 1.010 for the new yeast because it was a slightly lower reading than the 1.010 for the old yeast.
I can't say that I can detect any real flavor difference between the two half batches. The "old yeast" may be a little sweater but it is barely perceptible, I may even be imagining it. Both taste really good. I am certain that any taste difference could only be noticed by tasting each beer side by side. Tasted months apart, with different batches, you could never tell, there are many more other factors that could cause "identical" homebrew batches to be different (that is part of the fun).
My conclusion is that the "old yeast" was just fine to use and conversley, the "new yeast" provided no real advantage in the end product. The only advantage I see for the "new yeast" would be a higher chance of success and and lower chance of something bad happening.