I have just started learning guitar... I am a lefty, I play harmonium with my right hand... I don't know with which hand should I play guitar now... Which hand is considered as the main hand in guitar? The fretting hand or the other one?
I would recommend starting the conventional way regardless. If it isn’t working out, then switch. That way, if you can play then you don’t have to worry about mirroring things. I can’t say which hand is ‘main’ myself since I think that depends on style. If you think ‘requires more complex movement’ the in classical they are equal. While in pop that might be your left hand for chords. I start my violin students ‘normal’ at first. So far in have not had one who just couldn’t go at all.
If you can make the conventional way Work then you save yourself expense and effort finding specialty equipment in the future.
Jimmy Hendrix was a lefty who played a right handed guitar upside down because his dad didn’t want to get him a left handed guitar. Maybe you don’t have to go that far. If you feel like you have to, then maybe switch.
This has been aired several times. There's 'handed' 'fingered' and 'armed'. I believe that the fingers on the fretboard are the key. So, whichever set of your fingers are more 'dexterous' - or better controllable for want of another term - will be better for the fretting hand. That said, others will say the other set of fingers are important. True, especially if you want to play finger picking styles, but for strumming chords, the non-fretting fingers do very little, It's the hand/arm that does the work, and that is a different concept from fingers, which will always work harder, whichever style of guitar you play, until advanced levels.
If you do pick right-handed, there'll be far more choice in guitars, often at lower prices, and imagine being at a friend's house, but only playing lefty. You won't be able to borrow his guitar and play...
Play 'normal'. You REALLY don't need the complication of a leftie guitar. As you say, it's a toss-up which hand you consider does the most important stuff.