You can choose. Personally, I am using 4.3 quite a time for now. Although, do not use both in the same time.
Please note, all your settings should be compatible between 42<->43 just fine, you can use your configs (like muxes, channels, etc.) from tvheadend42 to tvheadend43 just by copying correct ones to the correct new folder within userdata folder.
I can confirm it is in the addons now. Unfortunately, after turning off TVH42, TVH43 doesn’t work out of the box, it doesn’t get the old settings from the TVH42 installation. I had to go through the tool I wrote some time ago. Maybe it helps someone else as well to save a lot of time.
I found that with 43 there were 35 muxes got stuck in the scan que on repeated rescans. Some of those muxes were essential for my needs. Thats got to be a sensitivity issue but unacceptable for my needs.
The funny thing is that 43 is still way behind the current versions of TVH. Every release seems to introduce a whole raft of problems which is the reason why people stick to the tried and tested 4.2 version.