They are somewhat different. As you can see from the specifications, the PL45 has a better close focusing distance, hence, is more suitable as a macro lens. The ZD50, on the other hand, has a larger maximum aperture. For that reason, it could be better to use as a portrait lens.
The lenses are compared below. As you can clearly see, the PL45, to the left, is the smallest lens. The ZD50 requires an adapter for use on a Micro Four Thirds lens.
But what about their performance? Quite obviously, the PL45 is much better in terms of autofocus speed. The ZD50 could not autofocus at all with the first generation of Panasonic G series cameras, and with newer cameras like the GH2, the focus speed is barely usable at all.
I have compared the bokeh of the two lenses. In my experience, the bokeh of the PL45 is more smooth. The ZD50 has somewhat sharper edges around the out of focus rendering of highlights.
Also, I've tried to compare their sharpness. My study, which focused most on closeup focus distances, is probably not the best. But I believe it shows that the sharpness of the PL45 is a bit better. But there is certainly room for interpretation of the results.
It is also a well known fact that the ZD50 has some Chromatic Aberration artifacts. I have explored that here.
I've tried to do a second test of their sharpness. This time, I focused on a distant object. I put the camera, the Panasonic GH2 on a tripod, used the lowest sensitivity available, ISO 160, turned off image stabilization, and used a two second shutter delay to avoid camera shake.
Here are the full images, taken with maximum aperture with both lenses. The images have been rescaled and sharpened:
To better compare the sharpness, I have cut out 100% crops from both images at similar apertures.
Here are 100% crops from the centre of the image frame. Click on the image for a larger view:
And these crops are from the lower left corner of the image:
And from the upper right corner:
First of all, we see again that the Olympus 50mm f/2 lens has more chromatic aberration artifacts. The artifacts persist until stopping down to f/5.6. This is three stops smaller than the maximum aperture.
You can see the artifacts as purple fringing around high contrast areas, e.g., around the scaffolding when there is a bright sky in the background.
The CA artifacts only appear when having a large contrast in the image. In a portrait photo, you're not very likely to experience high contrasts, and hence this is not likely to be a big problem.
Vignetting does not appear to be a problem with the ZD50. However, keep in mind that at f/2.8, it is already stopped down one stop from the maximum aperture. The PL45 gives some vignetting at f/2.8. We see this in the corner crops: They are darker at f/2.8 than f/4 and f/5.6.
When it comes to the general sharpness and contrast, I think that the 100% views show the PL45 to be slightly better. I think that the PL45 shows the most pleasing results. But both are certainly very capable. Unless you are going to print the images to very large sizes, I cannot see that any of the two lenses will displease you.
Some users of the Micro Four Thirds system are still waiting for the portrait prime lens. They could be unhappy with the Panasonic 45mm f/2.8 macro, since it does not have the sufficiently large aperture they expect.
According to the Olympus lens roadmap, they are going to release a 50mm macro lens for the m4/3 format soon. If it has a maximum aperture of f/2, like the Four Thirds counterpart, then it may perform better as a portrait lens than the PL45.
Rumors also say that Olympus will release a non-macro lens with a focal length of 40-50mm, and a maximum aperture larger than f/2. This has later been specified as an Olympus 45mm f/1.8 lens. If so, this will be an even better candidate for the portrait prime. We shall see quite soon, as these lenses are announced.