questions about tubes - to experts

I'm new in tubes (just recently bought first tube component ever), so I have some questions. I discovered Electro Harmonics company, how good are their tubes ?
Much worse them the best from what I had read here on Audiogon Western Electric, better them Sylvania, Swietlana... ?
For example is EH 300B worse them a few time more expensive Western Electric 300B ?
What is a difference between NIB pair of WE 350B and pair of WE 350B ? What NIB stands for ?