Panasonic 200mm v 300mm
I own both. The 45-200mm gets little use since I got the 100-300mm; I carry the shorter lens mainly when size and weight is the most important concern, or when I expect the 45-100mm range to be important.
The 300 is faster and sharper through their overlapping range, and it seems better to me at 300mm than the 45-200 does at 200mm. However, the 45-200 isn't a bad lens, especially for the price.
For birding, the 100-300mm wins hands-down. The extra reach is important, and it's a better lens. At a zoo, the 45-200mm can be good--sometimes you're close enough that the wider angle is useful. Also for travel, the 45-200mm is more versatile.