Hi
As yourself, I also decided to wait for the Nexus 9 specs before deciding. Now that I've seen the specs, I found nothing convincing about them (short or stock Android L and possibly 64-bit architecture), and just ordered the Galaxy Tab S 8.4, special deal at 299! My arguments for the Samsung tablet: much lighter (ca 30%!), better screen with higher DPI, micro-SD card extension up to 128GB, and last but not least the special deal which made it sooo much cheaper. Sure, the battery is smaller, but I definitely prefer the somewhat smaller screen and smaller battery in return for the much lower weight.
As for Android 5.0; I'm tempted to agree with StatuSChecKa, that Samsung is more likely to bring a new tablet to market with L before an update is made for the S... But if I ran the tablet division at Samsung, I would argue that the specs of the existing device are sufficiently superior to that of the Nexus 9, that providing a quick OS-upgrade of the Tab S line (and of course broadcasting that news worldwide) would make a lot of potential N9 buyers think twice, while at the same time securing the loyalty of everyone who already bought an S tab.
I'll definitely keep a close eye on the XDA forums in the coming months, hoping that some hacker-genius provides a workable Android L ROM for S 8.4 soon. Speaking of which; does anybody know if there is a stock Android KitKat ROM for the S 8.4?