Lower samling rates in MP3 mean lower quality. 48kHz is OK for telephone quality. Your best compromise between quality and size is in the VBR (Variable Bit Rate) encoded files, anything from V2 to V0. I like MP3
for what it is. And the V2 to V0 VBR files are excellent when encoded with the latest release of LAME (
LAME MP3 Encoder).
hydrogenaudio.org is when the LAME folks hang out and the fine points are discussed.
Film WAV files at 48kHz are in no way similar to MP3 files at 48kHz.
Cheers