I've been struggling with getting a longer lens for way too long. I have debated 300 f4 vs. 70-300 L vs. 400 f5.6 vs. 100-400 vs. 70-300 DO (not too much consideration) along with the TC issue (I have the Canon 1.4 but would need a Kenko to use with some of the above). You see what I have below.
Thought I'd settled on the 70-300 L after taking some shots of the moon with my 70-200+TC last week. The night was cloudy but the shots were not too bad with cropping post (BTW, the Redged legs are great but I'll probably up the ballhead in the future). I knew what I wanted and was just waiting for the right price.
Then, the 2 day Canon refurb sale comes up. As I was ready to buy a 70-300 L, I had a quick change of heart and purchased the 100-400! Should arrive soon. I truly hope there is no "buyer's remorse," but my thoughts were to expand my options with some IS and little overlap. We'll see.
100-400 is the best way to reach 400mm on a budget. About 2 years back I was in the same type of situation and went for the 100-400, happy ever since
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L IS, T28-300 Di VC PZD, Ʃ150-600 DG OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS