Thats a good question, I read a detailed technical review a few weeks back (if I can find it I will post the URL) that compared seagate and Western digital for performance in a NAS. The reviewer found that the two drives were neck and neck for performance on all the tests that effected a NAS performance.
Both these companies are reputable manufacturers that provide good quality products, I would use either in my equipment without concern.
In the end its more a matter of using which ever your most comfortable with, and if your unfamiliar go for best price / performance value.
One point I would make regarding photo collections, is that since a photo once stored on a drive remains in that location is not altered or changed, so disk performance is not as critical for this environment. So when choosing a drive the Western Digital RED are 5400 rpm, and are adequate for storage of large files. Typically the WD RED PRO are 7200 rpm and considerably faster for read / write, but in your case the usage is more write once, and read many times, so the standard WD RED 5400 rpm will be suitable.