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If small and light was the order of the day the 70-300 DO would be worth consideration.
The problem there is then you have to be willing to trade away a fair amount of image quality and yet pay ball park as much as the f/4 IS or 70-300L. But if you really want compact and light above all else and to extreme degree, than that is it.
The 70-200 f4IS is a dream travel lens and even with the 1.4x its razor sharp and you only loose 20mm compared to the 70-300.
it is, I'm testing to see if the 70-300L can replace it though. 300mm without swapping TC on and off is nice. So far:
the 70-300L compared to the f/4 IS appears to be:
noticeably better at 280mm (f/4 IS + 1.4x TC III);
better by a little bit at 200mm f/5-f/5.6 and worse by a tiny, tiny little bit f/6.3+;
(even in the center frame the f/4 and f/4.5 shots look a little darker than the either lens at f/5 so in terms of noise performance it is probably not a full 2/3 stop advantage over the 70-300L, maybe more like just over 1/3 of a stop, that said it does seem to deliver near the full 2/3 stops more background blur which is nice)
worse at 165mm;
a tiny bit worse at max mag ratio MFD (the f/4 IS is awful there wide open but once you are down to the f/5.6 the 70-300L can do then it seems to do tiny bit better, OTOH I get the feeling that the 70-300L is actually a 1/3 stop faster than rated there so that might change the results as they should be compared, maybe make it even);
testing more but it is probably worse at 135mm, similar to a trace better at 100mm and better at 70mm.
The 70-300L has a fair amount more CA on the widest end but a bit less on the longer end.
(The 70-200 f/4 IS appears to peak somewhere in the middle range, both just compared to itself and relative to the 70-300L. Rising up from 70mm or falling down from 200mm the f/4 IS seems to improve a bit. At f/5.6: Very tentatively, so far, the 70-300L seems to get better as you go 300mm to 200mm and then slightly worse as you head down to 165mm not sure the rest of the story. The Tamron 70-300 VC seems to get better just going merely from 300mm to 280mm but then seems to stay sort of put down to at least 165mm. The Tamron seems to hold it's own best either at the edges or when stopped down 2/3 of a stop.)
The 70-300L does weigh a bit more though. No focus limiter. It seems like the two focus at a similar speed on a 5D2, maybe a tiny trace faster for the f/4 IS on a 7D. AF speeds for the two seem similar on the 5D2 vs 7D though. All very close.
The f/4 IS + 1.4x TC III does focus more slowly, so 201mm-280mm that combo definitely focuses more slowly than the 70-300L. The 70-200 f/4 IS + TC combo seems to have same AF speed as the bare Tamron 70-300 VC. With no limiting ring the 70-300L is more prone to get lost though under bad scenarios I'd think, no clue why they left the limited off the 70-300L. Focusing ring is even smoother and IS is quieter on the 70-300L vs f/4 IS.
(The Tamron 70-300VC has more reach than either with its true 300mm vs 280mm for the f/4 IS+TC and a seemingly less than 300mm for the 70-300L, at least at most target distances. Then again the 70-300VC is blurrier than the 70-300L at 300mm so that mitigates most of the small reach advantage and even when reach limited the 70-300L results still tend to look a bit more pleasing and it's certainly better when you intend to use the entire frame at 300mm, at least at 25' distance, some say the Tamron does great at 300mm at 100' and over, not sure yet.)
Not needing to swap a TC on and off is joy.