I find the secret garden rate is about the same between 12-5 and 12-3. 12-3 takes less stamina and is better if you're only interested in manyus but 12-5 gives a lot more gold and seeds, and a higher level raid boss.

As for farming 12-5, I've found one really strong squad will go a lot further than several fairly strong squads. If the squad doesn't reach a certain level of power it will just be destroyed or crippled by the threestepper nodes. On the other hand one 100k+ team with Edelweiss and Lavender for sustainability and a slice type or 2 can clear the map on its own.