I call "West Side" anything west of the Lake Washington including Seatlle downtown as well as North Seattle and West Seattle (Alki). "East Side" is east of the lake:
http://maps.google.com/?ie=UTF8&ll=47.621438,-122.218437...The problem is that bridges and Seattle downtown highways get clogged for hours each day, effectively rendering east and west side distant cities. On the east side only Bellevue downtown gets clogged and even then it's fairly small and easy to avoid.
East side (Redmond, Bellevue, Kirklnad, Woodinville, Sammamish) are better for me and right about anyone else who lives on the east side. Microsoft has its offices in Redmond so you are likely to find a lot of customers. In fact you should become friends with some softies and ask them to hang your posters in the kitchens within the buildings. Nintendo offices are also nearby, fwiw.
The other thing in favor of the East side is that most exisitng shared spaces are on the west side, except I think one which is sort of expensive.
Well, I could be a bit selfish here so you will need to do a bit of your own research too. :-)