Not sure about the bang for the buck, as the 3.5 tend to be more expensive where I'm at. Worry not, tho, I won't be a cheapskate for when it matters.
As for the size, I'm looking for exactly the 2TB one, for which there's no issue in finding either.
As for the manufacturer, I'm leaning more toward Seagate, which have both types. I already have a few from them, one of which is a 2TB 2.5 HDD for a laptop, which I'm happy with. This one will be for my desktop.