The differences are only number of channels, size, number/types of antennas, telemetry and input voltage range. All will work just fine. The pricing is random and doesn't reflect any of that ironically.

There is nothing to stop you from getting the R2006GS except for it being a bit on the large side by modern receiver standards.

The GFE is a great receiver for racers - small, light and as noted, no antenna. However, it does lose a lot of the range for which Futaba's are legendary as it's only rated at 70 meters - kinda short for a 1/8th basher.