The only way that you will know which lens is better for you is to try them both. You mock people using our crap light as an excuse when in reality it isn't an excuse. It's a real life problem, I haven't taken my lens out in good light once, I can't change that. What I do know is that in the crap light it is a better lens than my old lens, other than that, I'll have to wait and see.
You've mentioned weight in another post, the lens is heavy and that should be beyond doubt, it's 6.3 lbs as far as I know. Is it possible to hand hold? Of course it is but I think it will take practice for some and it will build your left arm muscles up.
The first time I picked the lens up I thought that I had made a mistake, after using it I found that it wasn't too bad and it will only get easier to use with time but it will not get any lighter. I have little doubt that using it will become easier because I've gone through it with my previous lens.
I don't mock anyone. Never have and never will. Not my style. Crap light is a real issue. I generally don't even try to get pics unless it is to document a rarity unless the sun is shining and the light good. I went out yesterday and shot a good 300-400 pics of Hood Mergansers. They were in a dark cove and sun not direct. Were lots of harsh shadows. Light was not ideal where they were. Results were good but not great. Light had a good deal to do with it. The end result was that I deleted all of them.
I mentioned this somewhere. Got a chance to use a friends 300 2.8 is (version 1) very quickly a couple months back. Just 5 minutes at the side or a harbor shooting some flying gulls. Lens was lightning fast and pics were very nice. But that thing was so heavy that I would never, ever buy one. Perhaps a version 2, but no way version 1. I am not weak, and am quite fit but I would not be able to hand hold that thing for long periods of time.
Here is a shot that I got with it. Like the shot but handed his lens back to him and was happy to not be holding that thing any more. The Tamron felt like nothing compared to that beast.
As to the wait and see part. I am as well. Very much looking forward to shots with this and the Contemporary with good light.