For me, I wont go for third party brands. But you're not me.
So... between the two, not considering the optical qualities of the two (which I have no idea about), I'd say that the 24mm on the Sigma is probably more useful than the 28mm on the Tamron. There's 5mm more on the Tamron on the tele end, but I will not miss it compared to the extra 4mm on the wide angle side.
I'll take the 24-70. Agree with brian that the 4mm on the wide end will be more useful than the extra 5mm on the tele end, especially if you're on a crop body. AF speed and accuracy goes to the Sigma over the Tamron.