In my experience at present, comparing Chrome Canary with Firefox Nightly, SpiderMonkey and V8 both win on particular use cases now (sometimes quite dramatically).

V8 tends to win on stuff that depends on garbage collection since they have a generational collector, and SpiderMonkey only has incremental.

SpiderMonkey wins on some computation workloads because V8 still has a huge limitation where they often have to store floats as individual allocations on the heap (that's the best explanation I can come up with, at least; impossible to tell for sure from the Chrome profiler)

V8 also wins on some use cases that make use of new ES5 features like Object.defineProperty, because their optimizations do a better job with them.

IonMonkey is definitely a big improvement over the previous generation of SpiderMonkey, though. They're closing the gap.

P.S. It's a little hard to measure this stuff because V8 has some optimizations that happen to be perfect for benchmarks but provide a much smaller win for applications (Loop invariant code motion, for one). Nothing naughty going on here, you just need to be aware the numbers are lies.