YJD,
I would prefer three choices as in
- Sigma 70-300mm APO 1:2 Macro
-- similar to Tamron but without PF/CA
-- a bit more on price
-- sharpness is close, maybe a wash, to Tamron
-- I prefer Tamron on the sharpness based on my user experience
-- great on 1:2 close up - Tamron 70-300mm Di LD Macro 1:2 Macro
-- Hin's biased favorite in terms of sharpness,
-- caveats on CA/PF, some easily fixable in pp
-- great on 1:2 close up - Pentax DA 55-300mm
-- the best one in terms of IQ and focal range
-- lacks close up as compared to Tamron and Sigma zoom
-- price is more but worth the extra
I prefer 70-300mm over 100-300mm unless there is aperture advantage or superior advantage in IQ, which I can't comment as I have not used any 100-300mm. And I say that again
in case you also like DA 50-200mm, give that zoom a chance and
try it in the zoo and walk closer to the animals in the shooting. I love that under-rated zoom and I still miss it as it has the
most valuable zoom range from
50-200mm which I miss whenever I use DA* 50-135mm. I tend to think none of 300mm consumer zooms mentioned can give you the IQ as good as the DA 55-300mm but I think the DA 50-200mm can get very close to the DA 55-300mm for the shorter range under 200mm -- that is my gut feel.
Thanks,
Hin