I've owned an H140 for a few years now and tried out the X5 for a month. Eventually returned the X5 for three reasons:
1. I found the sound just too bright for my liking (used my ER4P with both players), the latest Rockbox EQ apparently allows negative adjustment but I've only tried the X5 with the original firmware, not Rockbox.
2. The subpack was the dumbest idea. I use the H140 to freely transport files back and forth between home and work computer, all I need is two mini-USB cables at both machines. May not be a significant factor to some, but to me it was a constant annoyance to carry the (pretty fragile) subpack and have to fiddle with it every time I need to transfer something.
3. The joystick control on the X5 (again, the original firmware, not Rockbox), while very nicely implemented requires a double click as opposed to a single click on the iRiver. Might be a minor thing to somebody who doesn't have to adjust from iRiver to Cowon, but to me it seemed an unneccessary and confusing at times.
Overall, it was the sound signature of the iRiver that was the major reason for my decision to stick with the H140, it sounds unbelievably good with the ER4P (they have the best EQ bass enhancement implementation IMHO). I just wish iRiver released a successor with a smaller form factor, nice storage capacity, large colour screen and DivX playback, I'd jump on this in a heartbeat even if it cost $500.
However, you should probably try and audition the X5 at some local store if you have any that carry it. Ideally you'd wanna audition both of them but nobody sells the H1xx anymore... Who knows, you may find the X5's sound to be a blissful sonic nirvana and not care about the factors 2 and 3 described in my post. The problem with the H1xx units currently sold is that they all are fairly old and even though the build quality is very good (mine has lasted for over 3 years used daily and still is going strong) you still have a significant risk of having some sort of a breakdown while the X5 can be had brand new.
Hope this helps and good luck!