My daughter was 5 when I had my son. I think that's a hard age difference at first, but as they get older, like now for me where she's 8 and he's 3, things get better 'cause they can play together more and communicate with each other.
My daughter was resentful of the extra time we had to spend with the baby and whenever we would say things like "Oh, isn't he cute", she would say "What about me?" like we didn't think she was cute anymore. Of course we always reassured her, but I think in the back of her mind, she always thought she'd been "replaced". That was when he was younger though.
I wouldn't say you shouldn't have another child, I would just say you have to be extra careful about the difference in how you treat both children. You have to give as much attention to your 5 year old as you can and reassure them they have not been replaced with this cute little bundle of joy. Also, you have to do a lot of explaining about why the baby gets to play with the toy first or gets to throw his food on the floor and stuff like that. In other words, a 1 year old doesn't know how to share, but a 6 year old does so the 1 year old (or 2 year old or 3 year old) might throw a fit to play with a toy that the 6, 7 or 8 year old is playing with so you either have to let the younger one have the toy first or put the toy up all together. That can create some hard feelings too.
At this point now though, my daughter is very loving to my son and she is always saying how cute he is when he does something or says something funny. She's protective of him too and watches out for him when they are at the sitters and things like that. She's a wonderful big sister and I think she loves her brother very much. Time will only tell if those feelings will last.
Good luck in whatever you decide. :-)