I think it's really all personal preference. You'll probably have a better idea once you get going with your first on whether you'd like another right away or if you should wait.
We have a daughter who will be 8 by the time this baby is born. So quite the age gap! Financially, we just couldn't have done it any closer together because of the extreme cost of daycare here. And really for awhile we considered only having the one child. But as fate had it - here we are! DD co-slept with us for quite awhile - and having a queen bed - it would have been pretty crowded in there with more than one! Now days she only hops in when she isn't feeling good or if we girls are having a "slumber party". I'm kind of glad for that because I don't feel I'd be comfortable with her in there at the same time as baby. Our DD flails and kicks around in her sleep and it wouldn't be safe.
But co-sleeping aside - I am kind of glad for the gap because we got to focus solely on our daughter for so long. Now she is at that age where she is coming into her own and going out and doing things with her little friends and going to sleepovers more often. So as she is growing up, I'll now have another little one to nurture - which is kind of nice. When DD is storming around during her teens, LO will still love me LOL!
But negativity aside - DD is old enough to realize exactly what is going on so it is like we are sharing the experience with her. She is as excited as we are! She'll be a great big sister and I know she'll like playing Mother Hen to her little brother and she'll even like helping out I'm sure. Also, having kids spread apart helps spread out expenses a bit too. Especially if you have to pay any sort of daycare fees.
Of course there are huge advantages to having kids closer in age too. I imagine they might have a tighter bond because they'd be playing together and maybe going to school together. It'd be a different type of relationship than one of a huge age gap would have. If you have 2 girls or 2 boys they could share a bedroom even. So that's kind of nice - for you anyway
All in all - it doesn't really matter. It's all what works for you and your family.