Great responses - I don't know why I need to say anything more, but since I WRESTLED and AGONIZED over this decision, I'm chiming in. I agree the siblings may or may not get along regardless of age difference, and there are pros and cons both ways. So it may be best to just think about your own needs. I was 36 when I had my first, and I worried about my age and didn't want to wait a long time for #2. But when I had my baby, I realized I wanted lots of time to devote to only her. I am not a good split-focus gal (ha! not the best trait for parenting!) so I did not want a little 2YO toddler running around while I had a newborn nursing round the clock. Ended up having #2 when the first was 3 years 8mos - and still nursing a few times a day! 3 year split is no guarantee that nursing is done, but I will say that at 2 she still nursed a LOT and at 3 she only nursing a few ritual times. The 3.5 year split is fine, but not as wonderful as I had thought. I thought 3 years was really good because they are a lot more independent and interesting in the outside world. True, but as someone said above, they are also MUCH more aware of being displaced, much more opinionated about how they want to do things, etc. Lots of constantly reminding her to be gentle and mellow with the baby. Ironically, I think my DD1 is more jealous of me than the baby - she wants to be DD2's mom instead!
In contrast - when I was pregnant with the first, the principal at my school said she had hers only 18 months apart and it was great. She got through the baby and diaper thing all in one fell swoop. At 8 and 9 years old, they are good friends. But she is a completely different personality than me, a real go-getter, push-on-through type. I am comtemplative and slow moving. I would have hated to have 2 toddlers in diapers! With my spread, DD1 was nearly fully potty-trained when #2 came.
Another friend had hers 2 years apart (unplanned) and said it was great the first year because the older one was pretty clueless about the younger, who was in the sling most of the time. Mom just carried baby and took 2YO out to parks and stuff a lot. Then the next year - when baby turned 1 - was hard!
Another had a 10 YO when she had baby twins! The first year, she said it was great because the oldest was old enough to really help with the babies. But when the twins were 3, the oldest was 13 - Yikes! Not a good combination to have headstrong preschoolers with a headstrong adolescent! So you can't figure it all out.
There's no perfect way, and even if one age spread is good for awhile, it can be a problem later. My best friend growing up had a sister 2 years younger and they went back and forth over the years, "hating" each other, then being best friends. As adults, they are very close.
So think about what you want as a parent in terms of diapering/potty training, nursing, having kids in similar stages or different stages. But don't think too much. Maybe just listen to your heart and just choose!