I believe iron is a little more forgiving for the beginner.
I learned on 1 and 2 cavity Lyman's. The much cheaper Lee's came out and I started using them.
Lot's of tricks to learn with them.
Graduated to a few NOE Aluminum moulds...3 and 4 cavity , still a few tricks in casting but the quality is off the chart.
I have never tried brass. And I do not have any 6 cavity moulds.
All of them have learning curves...the only way to learn is to cast boolits. You have a problem, ask for an answer. You can cast good boolits with any mould but you are going to have to put in the time over a hot pot learning what works and what doesn't.
The brand or material of the mould isn't going to make you a great caster. Iron is harder to damage, can take a bit of rough handling and might be preferred for the beginner, but a nice NOE can be a pleasure to cast with. Aluminum and brass is softer so easier to damage...be gentle with them.
Gary