The Nikon 18-200 is max 5.6 @200mm.
OP, I don't know if the Tamron has VR or OIS, but the 18-200 does. That has come in very handy. I believe the Sony body has image stabilization built in whereas the Nikons don't, so you have to get it built into the lens (VR) if you want it. I have a hard time getting good focus zoomed all the way out without VR on while handheld. That is probably what the Tamron would be like if it doesn't have any OIS built in. (I just skimmed the info, so it may or may not. I didn't see that it did have OIS built in.) The VR allows you to stop your lens down further than if you didn't have it, so you can zoom out more or hand hold lower light shots (to a point) with less OOF issues. It doesn't have a lock on the lens so some people have issues with lens creep. I carry mine around in an OpTech bag when I'm not actively shooting, so it doesn't bother me as it's not going anywhere. I like this lens for convenience and used it a lot in DLR. I didn't have to carry around my 70-300VR so that helped lighten the load, and I only needed to swap lenses for low light situations. It's only a couple hundred more, unless they've raised the price... You can get it on
Amazon for $692.99 and free shipping from Amazon.
http://www.amazon.com/Nikon-18-200mm-3-5-5-6-ED-IF-Zoom-Nikkor/dp/B000BY52NU