Make a short test file of your most challenging material and compare.
In my experience, I have found Compressor, when using the Frame Controls set to Better or Best, does an excellent resize. I also found doing the downscale first…and not trying to do it combined with the MPEG-2 encode, results in better quality. Make an SD intermediate, either Uncompressed or Pro Res, then encode that to MPEG-2. If its a relatively short piece, use CBR for the MPEG encode and not VBR.