I'm currently looking for a fast short zoom for my Nikon D90 camera.

I'm essentially looking for something to replace using my slow 18-200mm lens when shooting things like:
- indoors low light photography (e.g. school events)
- general walkaround use (especially when photographing the kids, candid portraits, etc) - maybe the lens to leave on the camera when I don't have a specific photographic thing in mind
- sea/landscapes (I'd probably invest in something like a 0.9ND and 0.9GND which I could use with this lens - I also just ordered some step-up rings (e.g. 62-72, 67-77, 72-77, etc) so I'd probably be able to standardise on 77mm fixed filters in future? (BTW I have a 72mm Hoya HD CPL already). My current UWA - the Sigma 8-16mm whilst being uber wide has a very bulbous front element, built in non removable petal hood and no front thread so kinda precludes filters - I'm however wondering if handholding a 100x150 lee GND filter in front would work or not - anyone know if this is feasible?)

I tend to like using shallow depth of field a lot when shooting people, etc (which is partially why I so love my 50/1.4 - but using that lens indoors can get pretty restrictive and even for group shots of children outdoors it can require me to go back a very long way). I also want to use something sharper than my 18-200 (not hard I know!).

So here is the rub...
- I could get the Nikon 17-55/2.8 (77mm thread 755g) for about AUD$2000 for Aus stock or probably as low as AUD$1350 grey market delivered or even about AUD$900-$1000 used if I can find one.
- or I could get the Tamron 17-50/2.8 Non VC (67mm thread 434g) brand new for AUD$340 grey market delivered which is about a third of the used Nikon or a quarter of the new Nikon
- Note: whilst the Sigma 17-50/2.8 is an option - even grey it would be $650 delivered and thus almost twice the price of the Tamron

Obviously the Nikon is arguably better - better build, possibly better IQ, faster and more reliable focusing (esp in low light)?, slightly longer - but it is at least 3 times better!??

So at the moment I'm leaning towards the Tamron (I have the Tamron 90mm macro and am very happy the lens - sharpness and IQ is great - getting that lens is the reason I branched out into further lenses after my 18-200 (which until I got the Tamron 90mm I was fully happy with until I had something better IQ wise to compare with - unfortunately ).