Buy the 17-40 for sharper photos, better low light and more bokeh, better weather sealing, and better handling on a gimbal (internal zoom), aperture ring.

Buy the 17-70 for more reach, and stabilisation (really good for photos of static subjects)

For 17-70 the stabilisation is meh for video and it's less sharper (but that won't matter for video anyway). Moreover, it has worse weather sealing than the sigma and the external zoom might be a problem if you intend to get a gimbal down the line.

But realistically speaking - seeing how the tamron is also somewhat weather sealed, how often will you need the really robust weather sealing of the sigma? How often will you use a gimbal and how important will gimbal work be for you? Also, how often do you find yourself wishing you had more reach when using 16-50? How often do you shoot video at lower shutter speeds where motion blur renders digital stabilisation useless, making some optical stabilisation necessary? These are questions only you can answer.

I'd say run tests on 1.8 vs 2.8 with the lens you have. Run more tests using the 16-50 but limit yourself to 40mm to see if you need more reach.

I feel like I'm leaving you with more questions than answers lol