The main differences between the CITI600 PRO, CITI400 PRO and CITI300 PRO apart from the power output will be size of each unit, the CITI300 PRO will be the most compact and gradually get larger for each model up, a pocket flash.
The length of CITI300 PRO is 19cm with 10 cm diameter and CITI400PRO is about 22cm in length and 25cm for CITI600 PRO. The weight of CITI300 PRO is 1.25kg, 2.1kg and 3.1kg for CITI400PRO and CITI600 PRO. CITI300 PRO is great to use on location and minimal weight and storage are essential.
The recycling time of CITI300 PRO is 1 second at full power while the recycling time for CITI400PRO and CITI600 PRO are 0.9 second at full power. The recycling time of all 3 models are pretty much similar.
Another big difference is that the more powerful CITI600 PRO will operate at 600Ws which will mean that you can reduce the power and will get a faster flash duration and recycle time when comparing this against the CITI400 PRO or CITI300 PRO at the same power. You will get around the same amount of full powered flashes per charge ranging from around 320 to 390 power, again if using the CITI600 PRO at the same power as the others you will get more flashes per charge. CITI300 PRO battery is cross compatible with PIKA200PRO (recycling time and flash duration varies when using PIKA200PRO battery due to lower battery capacity).
Another difference is that the CITI600 PRO will have a 38W modelling bulb, 30W for the CITI400 PRO and 12W for the CITI300 PRO. Another big difference is CITI300 PRO is a bicolour modelling bulb whereas the other is daylight balance modelling bulb.
The CITI300 PRO will also allow for control of 5 groups, whereas the 400 and 600 will allow for just 3 groups.
When using Smart bracket 2 (M-020030) on the CITI300 PRO, you will be able to move your flash closes or further away from the modifier to create different lighting effect. This feature is not available for both CITI400PRO and CITI600 PRO.
All are great flashes that have similar features but different powers and size, the CITI300 PRO is less powerful but much more portable, the CITI600 PRO is more powerful giving many options but larger and more heavy so portability is slightly reduced in comparison. However, the CITI400PRO is a happy medium between, whilst great for portability it still offers impressive power output at 400Ws