ok you dont need a 300 gallon tank for a dovii i have seen a breeding pair in a 90 and the male was 20''+ and the female was 13''+. as for what one you want they are both the same animal in different bodys. both will rearrange the tank how they feel. so just choose the one that you think looks the best. i have both and for my opinion on what one i like better i have to go with the dovii for sure. if you think that the male dovii get to big go with a female they have better color anyways IMO