David,
Both functions will perform the resize operation you are interested in. resizem is a Mapping Toolbox function. imresize is an Image Processing Toolbox function.
resizem returns double precision floating point output. imresize returns an output of the same datatype as the input.
The two functions use completely different codebases, so there are likely to be performance differences between them. Crude performance tests seem to indicate that imresize is significantly faster.
Hope that helps,
Alex.
