Though both pack 4K CMOS imagers, there are some major differences -- the higher-end PMW-F55 has a global shutter, wider color gamut and can capture 4k, 2k or HD video internally, while the PMW-F5 records 2k and HD natively with a rolling shutter (4k requires an optional RAW recorder, as discussed below).
ENGADGET: Sony goes Red-hunting with PMW-F55 and PMW-F5 pro CineAlta 4K Super 35mm sensor camcorders