I'm not familiar with Photoshow, but I tried Piwigo.
Piwigo is more for showcasing and sharing select photos. It doesn't manage photos in place. You have to import the photos into Piwigo (potentially duplicating).
Digikam is great for managing the photo library in place. In other words, you point it to your photos folder, and the changes you make in digikam like sorting, tagging, face recognition, etc., all that info can be stored in a separate database that digikam maintains. I don't like modifying the original files, or duplicating the files so I prefer digikam over other options (I would normally pick picasa desktop over digikam, if only picasa allowed keeping photos on a NAS and access through samba easily and allowed transfer of the info database to other computers easily, but unfortunately picasa desktop is primarily a single computer, local files kind of option, which I dislike)
Keep in mind that certain task like face recognition can be extremely cpu intensive and can lock up your container gui for a long time for 200,000 photos. I'd recommend testing on a small batch and doing the rest in batches.
200,000 pictures, whoa whats that in disk size