I have a BMW F650GS twin (so basically, the same bike as the F700GS). A friend of mine in Colorado had an F800GS. Couple summers ago he did a two week trip in Europe, and tried to rent a GS while there. Didn't have one, and ended up on a Tiger XCX.
When he got home, he said he dropped his bags in the hallway, walked straight to the computer, and put his GS up for sale. Week later he had a Tiger XCA in the garage.
Based on his (repeated) recommendations, I finally test rode a Tiger in October. I got back from the test ride, rode home on my baby GS, and immediately started planning the Tiger purchase.
I tried an XRX and an XCX. Ended up with the XC version, since it's got FAR better suspension. Currently, she's sitting in a warehouse in Vegas waiting for Haulbikes to get their shit together.
Test ride both. Be aware that both the Tiger and the BMW are getting decent refreshes for 2018. TFT screens and the like. BMW is getting a "better" engine - parallel twin is being replaced by a v-twin. Slight bump in horsepower. The big question mark for me is that they're moving the tank up top (always been under the seat). The BMW's biggest plus for most was that low centre of gravity - it'll be interesting to see how moving the tank changes that.
The only knock I've heard about the Tiger (offroad) is the tall first gear. Apparently being addressed in 2018. And of course the stock seat sucks (as does the BMW). And the stock windshield sucks. (As does the BMW).
Test ride both. Test ride them again. If one of them totally does it for you, there should be good deals to be had (not from Marin Speed Shop on Triumphs, though. AMHIK) and if not wait for the 2018s.