I think it will depend on your preference. The 300 is smaller than the 6i if you care about the size and also lighter by 1.4 kgs. If you view it side by side you would think that the 300 is just a 10 or 8 incher sub.
As I posted earlier the 300 goes a little bit deeper but the 6i is punchier.
The 300 is using NAD designed amp while the 6i has a BASH amp.
300 has 300 Watts, 425 Watts, 850 Watts (Continuous, Dynamic, Peak Power) while 6i has 225 Watts, 325 Watts, 650 Watts.
For me I would get the 300 for the same price and even if higher by 5%.
Try both as it could be used at the same time and decide for yourself.