HCI and graphics are other examples: a SIGCHI proceedings paper carries way more weight than a paper in one the HCI journals. And a SIGGRAPH proceedings paper is better than than a paper in something like Journal of Computer Graphics Techniques, although they've recently converted the SIGGRAPH proceedings into a special issue of ACM Transactions on Graphics (which is now more prestigious than other issues of that journal, because it's really a proceedings volume), mostly to avoid the confusion from other fields that don't consider conferences to be publications. Those two conferences actually have nearly exclusive prestige in their field, to the extent that they're the only top publication venue: if you're working in HCI or graphics you more or less need SIGCHI or SIGGRAPH publications to be considered a legit active researcher. They also tend to be more selective in peer-review than the journals are, with acceptance rates in the 10-15% range, while journals are usually more like 20-35%.

In AI I think it's a bit mixed, and maybe a US—European difference. Europeans tend to value journal publications in places like JAIR, JMLR, various IEEE Transactions, etc. more. Americans also value the best of those journals, but I get the impression that conference publications at places like NIPS, AAAI, and ICML are at least as good, and definitely better than a lower-tier journal.