If you are still planning a betta, then I would wait until the plants fill out a lot.
I've recently had a betta in a 55 gallon (with masses of planting) but he preferred to spend his time just in the densest areas of planting - that was obviously his comfort zone, and he had no intention of exploring the more open spaces.
In fact, his favourite spots were in the surface cover (watersprite) and amongst the thick dwarf sagitarius planting across the bottom of the tank. He loved the wriggling and the nestling and just hanging out in them like hammocks.
I have recently moved him into a smaller tank, where he is currently settling and exploring, but I expect he will make better use of the space there.
When left to their own devices, in large tanks, my betta always seem to just settle into a section of the tank and just use that. So I have come to the conclusion that a tank around 10-15 gallons is the max that they wish to 'own'. Of course some betta prefer much smaller than that, and I am talking about a fairly small observational sample, but I get plenty of time to observe them, and their little preferences.
: (Kham) mature setup; betta, purple rasboras, amano and wood shrimp. Heavily planted.
22litre cube tank
: (Tagawa); betta, 1 amano, yellow dwarf shrimp. Pond snails and 1 assassin. Heavily planted.
57litre cube tank
: ember tetras, 2 amano, 1 nerite, pond snails, 1 assassin, doing his thing. Heavily planted.
; honey gourami, cherry barbs, green neons, otocinclus, nerites, pond snails, amano. Heavily planted.