We've got a 4.5 year gap, for exactly the reason you're thinking about it. We were unsure about having a second, but eventually decided to stop thinking about it and just get on with it.
When we had DD2, I was worried that we had used up all our luck with DD1 as she was a really easy baby. DD2 turned out to be even easier. Than DD1. My maternity leave came at a great time - it covered DD1 starting school, so I was able to be around a bit more. We didn't ever have 2 sets of full time childcare costs.
My DDs are now 11 and 7. They have fairly recently started to play together on things like Minecraft, but otherwise don't play together a great deal. It means we try to have friends over to entertain each of them. Maybe that is different if you have a smaller age gap, but equally they don't squabble too much.
I also find that they need quite different things or we have to do something with DD1 and then do it again a few years later for DD2. That's beginning to get expensive and still isn't exactly fair as DD2 gets to do things twice.
I think DD2 has had to fight a bit to find her place in an otherwise established family and I sometimes feel that I don't know her as well as I know DD1. I'm not sure if that's just because I've know DD1 for longer of they are just quite different children and DD2 is a bit more private.
I think DD2 will have a particular good time when DD1 goes to university - that was my favourite time of my childhood as a second child.
A larger age gap stretches out the time you have a cute child for, before they grow out of the cute stage.
For my DDs it's been quite useful for DD2 to not be close to DD1 at school. My DD2 is lovely, but trickier to teach than DD1, who seems to have been a dream pupil at primary school. I think DD2 would be a bit in DD1's shadow if they were closer in age. Though I think teachers are more sensitive than they were when I was young and probably wouldn't compare siblings openly.
Some people might say it's like having 2 only children. I quite liked having one only child, so I don't mind having 2 only children, when it feels like that.
I've always said that the best age gap is the one you've got. Partly because you wouldn't otherwise have the children you've got, but also because there's not much point thinking otherwise.