i haven't used ignition, but citect, especially for its vintage, has advanced redundancy, scaling and client/server features. The alarm, report, trend, and IO servers can be run on separate computers or cores, it supports failover for redundancy, and if one server couldn't handle the load you could use additional servers to deal with the size if your system. I doubt many people need those features. The client and servers all communicate over TCP/IP, and it feels rather fast and efficient, maybe because it was written when computers were so much slower.
I have worked on a series of 9 plants that have a central control centre which all run citect. Although I feel that I have wasted a lot of time on random citect quirks in my life it is actually impressive how it is able to integrate the HMIs from those 9 plants in to one.
Citect also has a webclient which is just an activeX distribution of the display client but it isn't that different from ignition using java, you go to a web page, download some plugin and then you have a non-web based HMI going.
The citect process analyst is better than the trending tools in wonderware, wonderware historian, and factory talk. I haven't used the PI trending tool and didn't get much exposure to cimplicity either.