Does the federal govt monitoring the border become a friend of local ganja producers, effectively protecting the market for local growers?

I really doubt there will be much trouble moving weed between WA and OR. Even during the period when it was legal in WA and illegal in OR, OR police pretty much turned a blind eye to interstate "tourists" on suspiciously short trips across the border and back. Now that both states have legalized weed, this will be even less of an issue.

And the likelihood that you run into a federal officer while crossing the WA/OR border is pretty slim.

I guess the only times (in recent history anyway) that states vigorously defend their borders is when it impacts their tax revenues.

Are there big differences in how Washington and Oregon are taxing marijuana?

Not sure. I have heard, interestingly enough, that there are federal taxes on the sale of marijuana. Which is weird, given that the federal government does not legally recognize one's right to buy or sell marijuana. These federal tax burdens are quite high, if the media reports are accurate:

http://www.usatoday.com/story/news/nation/2014/11/03/irs-lim...

It's not that weird if you think about it - same line of reasoning is what finally got Al Capone. Legal or illegal, Uncle Sam always gets his.

There is a huge difference in taxes, WA has some ridiculously expensive recreational pot (http://www.leafly.com/stateoftheleaf). Oregon's medical prices are the lowest in the country right now, retail will be more but not that much more.

Right now rec shops in Vancouver, WA are benefiting from their proximity to Portland, but that's going to flip around once Oregon is up and running.

From marijuana.oregon.gov:

  Q: Can I take marijuana into Washington where they also allow recreational marijuana?
  A: No. You can not take marijuana across state lines.