Heya!
I haven't grown either strain before but typically you want to switch to flower when your plants are half your desired height. During the first couple of weeks of 12-12 indicas will stretch 1.5-2x their size and some sativa strains up to 3x. Keeping this in mind, using LST can help by tying down the branches to create a more even canopy and will make your plant shorter.
Once the plant has grown 4-6 nodes you could also top the plant, cutting the main stem to create two new main stems. This causes the plant to grow more horizontally than vertical.
Good luck and cheers!