I'm a little late to this Ketron vs. SD-50 party. I can't speak for the Ketron, I own the SD-50 and I like it alot. Now that I've solved my flash drive problem I love it! I guess it comes down to what do you want to accomplish musically. You know how someone says, "I want to buy a computer." What's the first question the computer guy asks you?
What do you want to do with this computer? Well for me the SD-50 was a perfect fit. I play gigs with my laptop and BIAB. I used to use two or three old laptops for backup but since PGmusic went all real tracks (not that I'm complaining) I needed a new fast computer. So now my problem was I can't afford to have a second computer for a backup. Thats where the SD-50 came in. It plays mp3 and midi files, I use both for gigs. It also runs about 4 hours on rechargeable batteries. I converted all my BIAB songs to mp3 plus the midi tracks I use for pop music. Put it all on a flash drive (which the SD-50 uses) and I now have a backup for my laptop. I was having a problem with my flashdrive because I was formating it FAT32 instead of FAT. Now that problem is solve this thing rocks! On some gigs I just use the SD-50 and leave my computer in the bag.

Dell Studio 16 Laptop i7 with 256 gig SSD
BIAB, Reaper, Encore, Sibelius
Roland SD-50