Why dun you save enough for either the 100-400 Canon or Sigma 50-500 before you decide? At $1K you can probably get a 70-200/4.0 from Canon, 70-200/2.8 from Sigma or 100-300/4.0 from Sigma. With $2K you can actually get any one of the above lens, saving your time to upgrade again. Of course these lens are very different in terms of their usage.
Be very sure of what you want to save $$$ from b&s for upgrading.
Is 300 really enough? Why the 300 mark? Will 70-200+1.4xtc do for your shooting style?
IMO, it this length falls just abit short for shooting bird, also for shooting the white tiger in the zoo (i am doing the head & shoulder kind of shot).
Canon 100-400IS, Sigma50-500 or Tamron 200-500 will be just nice for nature photography.
But this 3 lens may be too bulky for normal day to day use.
70-200 f2.8 or f4 is suitable for day to day use (may be f2.8 is abit on the heavy side). Stack it with 1.4x will get you 280, quite close to 300 mark.
A used sigma 70-200 f2.8+1.4xtc (becomes f4, step down to f5.6 for better image quality) just able to meet you 1k budget.
A used canon 70-200f4+1.4xtc may be slightly out of your budget. Max aperture will become f5.6, stop down to f8 for better image quality. Also it is quite difficult to find the tripod collar for this lens (about $190new, used are hard to come by). Though the aperture is smaller by 1 stop, remember it is much lighter than the f2.8.