You are likely to get different answers based upon personal preferences, but that is in essence what you asked for, so here is my $0.02...
As Jim mentioned, the pricing of each is different, so that could come into play. If you didn't want to spend over $300 for a ukulele, then I would look at Ohanas. If you wanted to spend around $1000, then it is Ponos. Then there is that mid-range of $350-$500 where they both play. If that is what you were looking to spend the next decision comes down to the other factors mentioned here... the build quality and the necks. When you get to the higher level Ohanas I feel their build quality is good. I have owned a few Ohanas and still have their most entry level solid wood soprano and have not had any issues. I did order them through reputable dealers (Mim & Elderly), so that is probably a factor in the quality. And I have heard/seen great quality in Ponos as well. When it comes to the necks, Ponos have thicker necks. They just do. This is both good and bad depending on your personal preference. I would chose Ohana because I do not care for a thicker neck. Mivo would chose Pono because of the thicker neck. Is there any way you would be able to try either/both before buying? That is always the best way to make sure what you are getting sounds, feels, and plays best. However you define best.