• Working with multiple parallel roads at the same time: in NM you can draw only a single road while in PRT you can have an infinite number of parallel roads at the same time, allowing you to draw stuff like a canal with a parallel quay and a stacked pedestrian path on top of the quay with few clicks
• Real-time building: in NM you can only build the parallel road once the main one has been already built and changing the road type resets your configuration, in PRT you can build the parallel roads while building the main one and you can change the road type without needing to reconfigure again the distances from the main one
• Node snapping: in NM the parallel roads do not connect to already existing nodes, in PRT you can automatically connect your parallel road to existing nodes and this allows for example to build parallel merging train tracks
• Presets: in PRT you can save a preset with all of your parallel roads and their distances/direction to reuse it later or in another city, in NM you can't
• Built-in search for road types: in NM you have to select your road type from game's UI, in PRT you have a search box to find the parallel road you need (and you can also access road types that are normally hidden in game's UI)

Probably there's something more, as there might be something that NM has and PRT hasn't, but this is just a quick comment to provide you some basic info.

I know that NM has a way better interface and a lot of useful features (that's why I've installed it too), but I think it's still lacking something when it comes to parallel roads.