Itzmered, sorry about the 100-400. That changes the game a bit. Well... for $1000 you can go off-brand (like the Sigmas) or make an even more difficult decision:
- Buy a L zoom like the 70-200 f4IS (used goes for about $1000+), or the 70-200 f4 (new $760) or a 70-200 f2.8 ($1000+ used) and make a compromise to achieve half the reach of the 100-400.
- Buy a non L zoom like the 70-300 which, in all honesty, is not as good as your actual 55-250, so I'd stay clear from those.
- Buy a dedicated wildlife telephoto like the EF 400mm f5.6L
Now, all this choices are not that much cheaper, and staying in the $1000 limit for a new lens is really, really tough. With that budget, the Sigma 150-500mm f5-6.3 appears to be the only option. My concern is that at 500mm f6.3 you are shooting wide open and the lens is probably not at its best, so you may need to go to even f8 to get good sharpness and then its not really that bright and the shutter speeds will not be ideal. The 120-400mm may also work, but I'm not sure about its Image Quality.
I cannot think of many other choices available to you... perhaps the 55-250 is actually the better option altogether until you can save for the $1500+ choices like the 70-300L, 100-400L or the 400f5.6L.
Rafa.