My only personal experience is with the Pentax DA 50-200 and 55-300, however I've done some research on the Tamron 70-300.
I've concluded that what matters is the particular lens you get, more than the design of the lens. Both my Pentax F 100-300 (the "bad" version) and the DA 50-200mm seriously outperformed my copy of the DA 55-300. So I would somewhat discount comments about performance, since they won't apply to your own lens.
Here are some considerations that I run across, but as I said I don't have personal experience with the Tamron, so take this for what it's worth, keeping in mind that performance comments may not apply to whatever lens you actually buy:
1. The Tamron is by all accounts a bargain.
2. The Tamron covers full-frame, if that's important to you for any reason.
3. The Tamron is bigger and requires a bigger filter.
4. The Pentax may display less purple fringing / CA, particularly at 300mm. This may matter more or less to you depending on how comfortable you are with removing this effect in PP.
5. The Tamron has closer focusing (at least at the longer focal length settings, via its macro feature.)
6. The Pentax has the ability to quick-shift from auto to manual focus. I have quick-shift on several Pentax lenses and have never had occasion to use the feature; the value of the feature probably depends on the type of pictures you take.
7. The Pentax DA 55-300 and Tamron include a hood in the price. The DA-L doesn't.
Question I don't have any knowledge of: the Tamron is reported to have a less than stellar hood mounting mechanism. Maybe someone with the lens can comment. I can say for example that my Pentax lenses including the DA 55-300 have all had a firmer hood bayonet fitting than my Sigma EX lens. But so did my Canon FD lenses, for the first fifteen years or so. So the durability of the hood mounting mechanism might be a factor, if we knew what it would be.
Paul
Last edited by tibbitts; 04-25-2011 at 04:49 PM.
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