I just want to know if this is the correct way to connect an Arduino UNO and 2 LED strips using an external power supply only (no USB).
Im using:
Arduino UNO
2 x LED strips are Adafruit Neopixel (60 LED each)
external power supply 5V, 3A
Looks good, except 5v goes to 5v pin on the Arduino.
Don’t plug in the USB cable when Arduino is externally powered.
Just keep in mind - "Vin" and the "barrel jack" are essentially useless. If you are using them, you are asking for trouble!
Connect your 5 V to the "5V" pin (that is after all, what it is for!) except when connecting to the PC via USB. 
120 leds will draw 120 * 60 mA at full white; your 3A power supply will not be sufficient for that.
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setBrightness(64); // 1/4 intensity (0-255)
In your physical wiring, it would be best if all grounds connected at the power supply negative terminal. This is done by running a wire from each LED strip to the PS negative. And the same for the capacitor and again for the UNO.
This is commonly know as a "Star" ground connection.
The most important thing is that your GND from your arduino is connected with your GND from your strip. If not you will get informations fails and your strip is doing crazy shit you dont want, like show wrong colour.
That is important.
Connecting the wiring as in the OP's drawing is the most important.
Problem is, now it appears that the data wire to the LEDs is running separately from the ground wire, particularly from the end of the first strip to the start of the second.
It is substantially more important that the data wire always travels paired with (bound to) the ground wire, wherever it goes.
Similarly, the 5 V power and the ground must travel everywhere as a pair. This is actually more important in this respect than a "star" grounding pattern.
The 1 mF capacitor and 470 Ohm resistor should be mounted at the start of the LED strip.
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