I absolutely love the look of the NW-E505. It's absolutely gorgeous. The OLED display down to the battery life. Everything about it. It'd be the first choice, if not for the software you have to use with it. Sonicstage is a bloated piece of crap, more often than not. It's slow, and is buggy as all get out. Otherwise, the Sony is a great device.
iRiver is not nearly as flashy, but it is a good device. Sounds good, and is built well enough. And it runs off of a single AA battery. So, that's always a plus. It doesn't feel as solid as the Sony, but is still very decent. The iRiver music manager, though a bit of a hassle, is nowhere near as frustrating as Sonicstage. So, yeah.
I've got two minidisc players, though, and have to use Sonicstage to transfer music anyway. So, I'm used to Sonicstage already. Therefore, I'd go for the Sony. Decent to good sound, solid design, crisp OLED display. And the estimated 50 hour battery life is attractive, too.