Olympus 6000 has a
10.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Olympus E-PL6 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VI processor.
Olympus E-PL6's sensor provides 6MP more than Olympus 6000's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 6000 and E-PL6 sensor size comparison.
As seen above, Olympus E-PL6 has a 8.0x Larger sensor area than Olympus 6000. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.