Well, my UWA are used in three field, sometimes i can add another field, so i want a lens that can serve me for more fields than a lens that is very strong in one field and it is so so in another fields, so that i put f2.8 as priority.
I shoot landscapes, i shoot nighshots, i shoot long exposures, and i am really really serious about getting into Astro, and i am planning if i travel again after long time so i will shoot more of astro next time, i have Samyang in this area, but this lens has its pros and cons, and i also don't depend on one lens, and sometimes compositions control me, primes killing me sometimes, so that when i travel i prefer zooms, and maybe i won't carry Samyang with me, so i always put my 3 main zooms in bag then few more primes or extra zooms.
I already ordered the Tamron and waiting to receive it, so 16-35 is out of question completely, but in fact i has a plan to buy 11-24 in the future later, this year or next year who knows, but i was asking about, if i will buy that Canon 11-24, is it more compatible or logic with Tamron 15-30 f2.8 or Canon 16-35 f4? I can use Tamron and sell it to buy Canon 16-35 f4, but i don't feel that i would like to have lenses with only f4 with or without IS and no f2.8, and i am not looking for 20/24mm or longer, i have TS24 so i am not thinking about longer FL, that 15mm is really wide, but from what i shoot sometimes i really like to go even wider than 14mm, and that Samyang 14mm has that ugly converge[distortion] that i many times hate to use it, TS17 is wide but not that much wide so i keep using shift a lot, but if i travel and i shoot landscapes i may need zoom for compositions, and also for astro, and most of the time i am in move so i don't have time to change lenses as i will do that with many primes, when i was traveling in the past i was using 16-35mm a lot and i wasn't at 16mm 100%, and many times i was in situations where i wanted wider than 16mm itself, but last time i traveled was in 2009, and i bought that Samyang 14 and TS lenses after 2012, and i sold my 16-35 2.8 mk1 so wanted to have a replacement which was either Tamron or Canon 16-35 f4.