hi,
i'd suggest to always repeat the depth that you have achieved many times in order to continuously improve your technique, relaxation, streamlining, equalising, etc. with this increase of skill level going to that depth will become easier. you will also gain a lot of diving experience which is very important to be able to freedive more safely. without good experience of how your body reacts, how you feel, how much effort you make, how the daily conditions might affect you, you'll more likely risk unwanted surprises like black outs or injuries. here is where this cliche of freedivers looking into themselves while scuba divers look outside comes into play. also, your relaxation and enjoyment will increase as diving becomes more easy.
regarding increasing depth or time first, i recommend you do repetitive dives with increased bottom times first (hangs, you don't move around). if you do this you change only one variable of that dive, time. if you were to increase depth first you would change two variables, time and effort. if you have become comfortable on dives with increased bottom times start going a bit deeper.
always increase in small increments, be conservative, and pay good attention to how you feel. the 'limit' varies for any given day for any given dive. a dive that one day feels totally easy can be very difficult on another day. take your time and safe and sustainable progression is practically guaranteed.
cheers,
roland