Sigma 120-400 or Sigma 150-500?

[SkyBaby]

I am saving up for a new lens and not too far from being able to afford one. Well, new to me that is, as I will be buying used locally so I can save some money and can check it out first. All I have right now is the Canon 28-135, which performs well and its sharp enough for me, but I need more reach. I am aware the Canon 100-400 is sharper, but its also out of my budget, even used, as much as I would love one. Either Sigma is in my budget.

With the Bigmos, it's f/5-6.3 but I get more reach than the 120-400mm, but the 120-400 is slightly faster at f/4.5-5.6 My question is, should I go for the shorter but slightly faster lens, or the longer but slightly slower lens? Both cost about the same on the used market and perform similarly image quality wise. I also know the Bigmos is a bit heavier though. Would the extra weight be that noticeable? I have handled the 100-400 on my camera in the field before. Is the Bigmos all that much heavier?

Thanks for your help!