RE: extensions and bases
I was looking for a skinny extension not that long ago and was considering Streamline, but I wanted to also use the latest Chinook SS Europin universal which I had already acquired. What I found was that the Streamline extension couldn't capture and retain Chinook's pin. However, the Streamline extension worked perfectly with a Streamline universal. So, if you want to mix and match, it's a good idea to test fit the components before purchasing them. Needless to say, I ended up buying the Chinook extension.
Yet, if I remember correctly, you're a dedicated 2 pin cup user. I seriously doubt that there would be any interchangeability problems with the 2 pin cup design.
In my opinion, I think that both the Streamline and Chinook products are pretty solid overall. In my mind, it's a carbon versus aluminum thing. With a carbon choice, it's strickly Chinook. On the other hand, if one was to select aluminum, I would clearly choose Streamline over Chinook because I think the former offers a more robust design in most respects (less plastic).
Another good focus point to weight would be the pulley configuration. Streamline's "quick 6" pulley configuration is super solid, but it requires doing things a bit differently due to the fact that the orientation of the pulleys is in line with the sail sheaves. Chinook's pulley configuration is more standard with a rotated 90 degree orientation to the sail sheaves. In reality, a lot has to do with the type of winch one uses (back problems requires that I use one). For years I have been using a lever design called "Rig It Right", and I found that it didn't work that great with Chinook's extensions, hence my preference for the "quick 6" design. Yet, if one is using a Chinook type winch where the line rotates along a shaft to tighten the downhaul, I think either extension design will work well.
Lastly, one might want consider price. Streamline's aluminum extensions are priced up there in parallel with carbon extensions, so one would paying a premium compared with Chinook's aluminum extensions at probably half the price. A pretty strong incentive to go Chinook. Really, again, it comes down to this carbon versus aluminum thing.
Too bad that Maui Sails doesn't offer extensions as one of their products. Given their incredible carbon boom designs, I know that it's very likely they would offer a dynamite extension design. It could be that they might have something in the works. Something worth checking.