Here is my understanding, based on a conversation specifically about this subject during the AEA G3X class I took a couple of weeks ago:
The advantage of the GTX45 for ASB-B in vs. the GDR39-R is that the GTX45 has three RS-232 outputs allowing you to hardwire it to up to 3 GDU displays so you can have the ADSB weather and traffic on any/all of the displays in a 3 screen panel. Also the GTX45 uses a single antenna instead of requiring 2 separate antennas for in and out that is required when using the GDR39.
The GDR39-R only has a single RS-232 output, so can only be hardwired to a single GDU display, allowing ADSB weather on just a single display. (The GDR39 sends traffic over the CAN bus so that is available on all of your GDU screens.) While you can use Bluetooth to connect the GDR39 to the other GDU displays, you are limited to the number of Bluetooth connections and so could be prevented from connecting to an portable (iPad with Foreflight for example) if using the Bluetooth to connect to 2 GDU displays. Also, the hardwired connection can be more reliable than the Bluetooth connection in some circumstances.
The GTX 45R also has 2 Bluetooth connections, which can be used to send weather/traffic to portable devices if desired.
Since I am planning a 3 screen system, this was enough to cause me to change my plans and switch to the GTX 45 for my panel, even though I already purchased and installed an ADSB-In antenna in my tail cone for the GDL 39R. Guess I'll close off that hole with a piece of aluminum and a few rivets.