That's a good question. If you get the TG77, it has this cool "bar-graph" display that indicates activity on each MIDI channel the TG77 is receiving.
If you go SY77, it doesn't have the "bar-graph" but it has a really nice 16 track sequencer, 720k MS-DOS compatible disk drive(you can download patches from the internet and load right from the PC diskette), and a second assignable modulation wheel.
I use the extra mod wheel to control volume on my synth modules.
The TG and SY share the same PCM samples and patches. They sound identical.