I always super on top. Here's an extract from one bee association website, which is consistent with what most beeks do:
Supers are usually added on top of the previous super. This is the easiest way to put on supers as less lifting is required and you can easily check the existing super to see if another is required. Bottom supering involves lifting one or more nearly full supers off and adding the new super directly above the brood nest. It demands a lot more work, which is only warranted if supering has been delayed and combs in the top super have been completely capped over.