70-200L 2.8 IS Mk2 + 2x TC vs. 100-400mm f/4.5-5.6 L IS

Hello,

I have 50D and Canon EF 70-200mm f/2.8 L IS II USM Lens. Few days had opportunity to shot natural wildlife (birds, forest animals), but understood that there is not enough focal length. There are these options:

1. Add Canon 2x EF Extender II (Teleconverter) Canon EF 70-200mm f/2.8 L IS II USM Lens. Price - $230;

2. Add Canon 2x EF Extender III (Teleconverter) to my existing Canon EF 70-200mm f/2.8 L IS II USM Lens. Price - $470;

3. Buy used Canon EF 100-400mm f/4.5-5.6 L IS USM Lens. Price approx. $1400;

4. Buy used Sigma 50-500mm f4-6.3 EX DG/HSM lens. Price approx. $750.

Which option you choosed? Thanks in advance!

Based on the great comments about the combination of 70-200 2.8 IS II and 2x TC III I finally own them and found excellent results. However, if you want to save money you would better go with 2x TC II. I did own Canon 100-400 f4 before and to be honest it could not be compared with the above-mentioned combination.

works nice with the 2x III, focus not as fast but still ok

Hire a lens for the trip that way you could go even bigger ! 300 2.8 & 2x telecon or even a 500 f/4
.............just a thought !

I recently bought a 100-400L. It is very sharp wide open at 400mm. I think that is the best option here. Sigma is VERY heavy. A 70-200 II w/2xTC would lose some acuity; how much, I don't know. One thing I noticed about the 100-400L, it is relatively slow to focus compared to my 85 1.8 USM and 70-210 f/3.5-4.5 USM. In really good light it is better. I am not complaining about it; there is a LOT of glass in this lens. Stabilization may be part of it. I have not tried it with the IS off, yet.

I was more worried about sharpness at 400 than focus speed. I am happy with the 100-400 and would buy it again.

Well. Bought slightly used 2X II for 160$. I think it was bargain, so I couldn't resist.
First experience - focus gets fuzzy if there is small contrast ant lens sits on 2 m focus setting, when you need to get focus on 500 m object. But once range is in logical limits, AF works quite well.
Regarding sharpness - there is a hit. But quite usable from F7.1-8