The Horizon probably has more capacity but the WSM is much more convenient in terms of the constant effort required to do long cooks on the offset. I started with an offset and picked up a cheap WSM off of Craigslist. The offset is showier if you are having a big party, but the WSM is better if you are doing small cooks for the family. I use both, sometimes at the same time. The offset is better (IMHO) for stuff like ribs and chicken that need to be tended, turned, or spritzed (if you do that sort of thing) because I tend to get temp spikes in my WSM if I open it very often.
BBQ Apprentice - Old Country Brazos offset, 22.5 WSM, Various gassers, portable grills, etc.