What's a fair comparison? You can quote specs, and you can quote preferences, but the former are only part of the story and the latter are personal.
Best anyone can do is tell you honestly what they like or dislike. Best you can do is see which of these preferences gel with you.
Then, most importantly, go and see/play with both devices yourself, because what's comfortable for one person may be awkward for you, what one person finds garish you might like, etc.
I've played with a One, not yet seen an S4. I know I like the One, don't yet know about the S4 because I've not seen it - I've read pretty much everything, but not seen it so don't know. I liked the S3's specs, but it didn't fit my hands well, but the S4 is slightly smaller and that may make the difference (the One X, which is the same width, I could cope with better than the S3). In the end there's no substitute for using the real thing.