Hi Hugh,
Strictly speaking resampling and filtering are different operations, though the end result is similar.
With resampling you are combining adjacent sample bins (eg pixel columns) to produce a spectrum with a different bin size. Unless resampling using a whole number of bins, this involves some degree of interpolation eg in ISIS, bilinear or spline. This happens to some extent in the background in any case to match the bins when overlaying the differently sampled measured and reference spectra)
With filtering you preserve the original sample bin size but smooth the differences between adjacent bins using some algorithm (eg boxcar, low pass, spline, gaussian etc)
In general it is better to filter rather than resample as resampling can produce artifacts compared with filtering. (Interestingly ISIS actually up samples spectra (ie interpolates to a smaller bin size) before wavelength calibration to minimise these effects )
Cheers
Robin