Which lens for wide field astro photography?

[Dinozorr]

Hello,I'm not sure if I've posted this in the correct place but here goes.
I'm looking at two second hand lens for my canon 5d mk1
One is the canon 17-35 f2.8 and the other is a canon ef 17-40 f4. Both around £425 ish!
Could some one advise on which would be best. I'm thinking probably the f2.5?
Thanks for any advise!

 

[Vesalius]

I'm very new to the topic myself but from what I've gathered the wider the aperture the better, so it would be a no brainer in this situation.
for stuff like milky way astrophotography anyway.

*holds on for actual advice*