Gimp is 8 bit last time I looked, where Photoshop was 32 bit, and Photoshop linked with bridge and RAW which allowed batch processing, and in the main it is the batch processing where Photoshop shines, I can do most with Rawtherapee, however I had got use to how to used Photoshop, but with Paint shop Pro 9 now loaded, I can get by, but I have a 500 page user guide to wade through to work out how. Also have Photomatrix to help process from RAW to 8 bit, but it is slow, I take a photo and process with Photomatrix, to get the tone mapping, then straighten with Rawtherapee, then use Paint shop to colour correct.
Still not found how to do vignetting, it was so easy in Photoshop RAW.