Well I can't say I have experience of both but I'd bet a bigger age gap is easier.
Mine are 4 yrs apart not through choice but when I look at others with smaller gaps I think it was an accidental blessing.
DD was at school when DS was born and I was on mat leave for her reception year.
I could have naps in the day and didn't have to drag a newborn around to toddler activities.
She was independent dressing, toilet, eating etc and could wait a bit if I was busy with the baby and play on her own a bit. She was reliably sleeping unless ill. No way I could deal with 2 lots of night waking.
There was never any serious jealousy issue.
She was ( and is) actually a great help to me entertaining him and watching out for him. I was afraid they wouldn't be friends but they have a lovely close relationship, play together all the time and I think bigger age gap/ different sex means less competing for the same toys etc.
I am most amused at my friend who has 2 girls with a tiny age gap and has exact duplicates of nearly every toy as she was so sick of the fighting and not sharing.
We haven't so far had a lot of problem finding shared interests for family days out etc and its quite nice sometimes for each parent to take them separately to different activities.
Sorry OP think your friend might be right