The primary differences between those two receiver models are the software.
The GR-12 has additional curve mixers digital switches and supports serial outputs such as SUMI, SUMD, SBUS, SUMO, and SUMD3 support for the mz-32 to enable signal output above 16 channels.
The GR-12L also provides support for SBUS, SUMD, SUMD3, and SP2048 serial protocols, and for those flying copters, you can designate the output channel of the RSSI signal for your flight controller.
Fundamentally there are no differences between both receivers unless you need additional mixer functionality the GR-12 has.
If you are looking for an all-rounder six-channel receiver with good range, telemetry, digital serial output at an affordable price, the GR-12L is the receiver to choose.