There for sure "can" be a difference in internal vs external antenna. There can also be no difference...it all depends on the design and the use case for which one is more appropriate. Some external antenna are nothing more than for show.
The overall radiated energy is what is approved and in theory no different across the different antennas. What is different is how that energy is radiated. The higher the gain antenna....usually the more narrow or more directional the beam becomes. So if you have low gain antennas on an AP sitting in the middle of the house, there is probably decent signal on the floor below, the floor above, and the floor it resides on up to a certain distance. If you put high gain uni-directional antennas on the same device, the signal strength above and below will probably suffer, but the range will be better on the same floor.
I put higher gain antennas on my both of my RT-N66U and got mixed results. The signal strength and speeds between the two (one as an AP and one as a Media Bridge) were greatly improved. However I now noticed I had more dead zones in various parts of the house.
Also keep in mind just because your router can blast a signal a long distance doesn't mean your portable device has the same ability to blast one back. You didn't provide nearly enough detail on your use case and environment you want to cover to really provide any guidance.