External antennas do offer the option of upgrade. They also can be angled for better coverage to above and below the router.
They do require more space and may not be as aesthetic.
If you are buying an AC router because you think it will magically fix any 2.4Ghz problems you are having, you may be disappointed. It may help or it may not. If you are buying an AC router because you think you will achieve the gigabit transfer rates implied by the naming, you WILL be disappointed. If your device(s) support 5Ghz and you didn't have a dual band router before, you may benefit depending on the distance and obstructions. If you had a dual band wireless N900 before, you probably won't see significant difference.