ZP - There are other's on this board more qualified to answer those questions, but I"ll give you my thoughts.
The ECM8000's, being a small diameter OMNI, will work very well in just about any application you would reach for an omni on. As omni mics, the pick-up pattern is well, omni directional, meaning they pick-up a sound source from just about every direction, including room reverberation.
Aside from O.H's I have found that they work well for small ensembles, but usually in conjunction with other SD condensors.
I have even used them on grand piano, and the results were..... acceptable. I think any time you have a condition where there is a lot happening, musically, or you want to capture some room ambience, the ECM's excell at.
The 603's, being a small diameter condenser mic can be used where detail and clarity are essential. Most common uses I have found are delicate acoustic instruments. While the 603's can be very good at this, my preference for those types of applications is the Shure SM81's. However; the price difference between the 603's and the SM81's may not warrant the purchase of the latter if you're on a budget.
For an all around, small diameter condenser microphone, the 603's are fast becomming a studio work horse, and would be a nice addittion to any mic locker.