phosphomolybdenum blue 660nm versus 880nm which wavelenght gives the best reading

Depends on your application.  If there is no background interference
with the phosphomoolydenum blue test (i.e. you don't have something
else absorbing at that wavelength.) then the 880 has a larger
extinction coefficent, or in other words is a more sensitive test.

Some background information can be found in the following paper and
Reference # 32 mentioned at the end of the paper.

http://www.shd.org.yu/HtDocs/SHD/Vol68/No1/V68-No1-08.pdf

Was this question asked before? I seem to rememeber a similar question
in the chemistry section