Those looking for a short answer, yes it's worth the money on the used market over the Tamron 70-300. It's actually not much if any sharper but it has far better colour and especially contrast. It's also stronger wide open and is faster than f/5 quite a ways through the range.
AF is slow and AF-C a complete waste of time. The 55-70 range is just a bonus, I'll be keeping this one.