Despite my complaints about Nirvanahq's UI, I echo what
@mcogilvie says and will be sticking with Nirvana as well. For all my kvetching, I've realized that there are features I like about Nirvana, and despite its flaws it serves well as a trusted system. Moreover, the grass always seems greener on the other side, but there are things that annoy me about Todoist and Nozbe as well. All three options are good. None are perfect.
Finally, the amount of work it would take to move my lists to another software solution in my experience takes the better part of a day. That time could be better spent... getting things done. (See what I did there?)