no please don't sell your d50, it's such a wonderful recorder!
the zoom might be good (i doubt it but it is possible), but it's not worth it to sell your d50. if you were a freelancer and pay taxes and do investments you've probably already would've paid of the d50.
if i were you i would invest in a clean and simple 2 channel preamp and a good bag. buying ...
I can vouch for JrF: http://hydrophones.blogspot.se/2011/05/c-series-pro-contact-microphones-new.html
I never leave home without them. Using an H4n.
They are used by BBC, which according to me is a good reference.
Jez riley French is an established name in hydrophones and also contact mics, especially for people who find Trance's offering "on the expensive side".
Also see this: Can you suggest a good contact mic for sound design? (average cost of proposal would be much appreciated)
"Head to head" internal mic recording test of Sony M-10 vs. D-50.
I have owned a D-50 for about 8 years, purchased the moment it came out and used about twice a week. I did have a period of non-use but stored it with fresh batteries and did not get the "dead" problem I've read about. I've owned an M-10 for about 2 years and used it occasionally.
the D50 is a fantastic recorder if you can afford to buy it over the H4n, the mics don't sound any better and the noise floor fairly similar with both using XY electret mics which always exhibit noise, the D50 is a tad quieter but I doubt in real world scenarios you'll find yourself in a quiet room recording nothing with built in mics.
the preamps in the H4n ...