hi gadrian, Wah! i've been eyeing the same stuff as u!!
i was really thinking of getting the 12-24 sigma as well...
1. coz cheaper
2. coz it can be used on full frame as well...
however, im a user that absolutely requires to protect lens with a good uv filter, and the 12-24 sigma doesnt allow that... it doesnt even have facility to put in cokins or any screw on filters... i often use GD and ND and polarizers... so bo pian.. and im also super paranoid abt scratching the lens... the sigma 12-24 mega POPEYE bulbous lens looks waiting to kana scratch...
thats the main reason why i think i'll have to go with the 12-24 nikon instead....
i also read from DPREVIEW, that HENRY GOH feels the 28-75 is brighter, better resolving power too than the 18-70... but its slightly larger/heavier, and not as goood general purpose as the 18-70....
he also posted a fantastic studio shot with the 28-75. looks good...
but since im getting the 12-24 anwytay, i think i'll probably go with the tamron...