I do own an Protool DSG-AG 125 and it comes in handy when levelling concrete, grout or plaster. But always use a VAC because the dust is terrible.
Both machines use the same grinding discs. And both of them are now available under the Festool brand. TTS probably find both machines have their own purpose.
The DSG does have a lower wattage of (1,400 to 1,700) and a higher rpm (11,000 to 7,700). But if that is better I don't know.
The dust cap of the DSG can be disconnected and the machine then can be used as a normal grinder. I don't think this can be done with the RG 130.
Although the RG 130 once was a normal AGP 150 grinder under the Protool brand.
You can always connect with Festool and ask them the differences between both machines. Especially when you have a purpose in mind.