Well as far as I can tell, the 775 is no longer made, so you can't get a new one. That's your first major difference.
Other differences:
* RBX375 has two MM-inspired humbuckers. The 775 has two soapbar humbuckers similar to the current TRB series pickups.
* RBX375 has a conventional 2-band EQ with volume, pan, bass, treble controls. RBX775 has master volume, master tone, and active gain boost. I've heard of 775s having 2- or 3-band EQs, but have not seen a picture of a 775 with this config.
* RBX375 was designed for a lower pricepoint, even given that it has a more flexible preamp. The 775, when it was produced, was among the highest-level RBXs made. Component and build quality might therefore be less on the 375. Yamaha's main method of price-cutting is cheaper components, while their build quality, even on lower-price instruments, is usually good.
* RBX375 is not available in dark blue. RBX775 is not available in metallic red. Both ARE available in black and silver.