They do different things. the ?i series is mainly intended to debug and
improve the compilers:
it emulates only so much of the architecture and system, enough to get the
compiler and libc going.

5e's a nice bit of work (except for the limitation to little-endian host).
the emulation of /proc was interesting.
5e is more useful if you're trying to debug an arm program on an Intel,
hence the interest in
emulating system calls. it's probably not as useful for compiler debugging.
when debugging the compilers it's convenient to run 5i with $ti :r and just
let it go,
rather than interacting with it by setting breakpoints etc through /proc.