below are my summary after playing with them for quite some while ago.

Sickma focus real fast but does not guarantee pristine image quality, tammy in the other hand, offers quality instead of focusing performances.

Nikon offers both at the cost of 3 times more expensive.  Canon offers an over priced lens at this FL range.

I really wonder why Tokina never popular in Malaysia.
Their built quality are top-notch and yet it's wider than T1750mm aswell.

If I have the budget for sure I'll go for Tokina as the built is compareable to Nikkor ones.

which is why i'm looking for one since i helped my friend cover his wedding and the outcome is not so favourable with T17-50. SWM ftw