Panasonic 100-300 a good lens?

Not a good idea in my opinion. I have both lenses. The 45-200 is a decent lens at best. The 100-300 is very good across the entire range. It's not quite as good at the long end as at the short, but that's true about pretty much every tele zoom. The 100-300 easily outpaces the 45-200 at all FLs they have in common. It's also faster at these common FLs. It's bigger and heavier than the 45-200 but not by a whole lot (126 mm, 520 g versus 100 mm, 380 g). The 45-200 is rather clumsy for what it is. The 100-300 isn't. The 100-300 costs more but is well worth the extra money if you want best quality as well as maximum reach. My 45-200 is ready to go for sale. My 100-300 is not.