I like the magical 3yr age gap. By 3yrs most can be reasonable, do quite a bit for themselves and almost if not toilet trained.
My boys are great mates and both have had the doting part when baby first arrives home. They are great at including and unfortunately giving in to DD (making very much for a little princess in the making).
Everyone will be different and have a 'magic number'
I have a GF who had her first two only 15mth's apart and found it tough going as her first one didn't walk until 18mths, so she had to be super organised come feeding time.

ETA: Sorry, didn't realise you wanted both perspectives, I can only give the 3yr age gap perspective as each is a 3yr gap. On the plus side, I was able to go back to work inbetween kids and earn some income