Both recorders offers similar features.
Zoom MRS-4 This recorder is really quite full-featured, although due it its age is limited in the size of the SD data card you can use. It can record two tracks at once. It stores alternative takes for each track, and you can bounce tracks to effectively make a recording with eight tracks. You can punch in and punch out to record over short sections of any given track. It can use batteries or the included AC adapter.
TASCAM DP-006 I haven't used the Tascam much; I ordered when I bought the Roland amp to get a discount for purchases above a certain amount, basically getting the recorder for free. The big advantage over the Zoom is that it has built-in microphones. It's also smaller and lighter for slightly better portability. The big disadvantage is it's battery-only, so limited life.