Surely the answer to your question cannot be answered unless we know what your application is; are you looking at a one off project? to standardise for a complete factory installation? will you have different types of applications? how many I/O?, 3rd party interfaces, eg. HMI, SCADA, etc? the list is endless.
In my experience, in the UK, Mitsubishi tend to be cheaper and have a bigger instruction set, particularly for machine control, sequencing, etc. (having said that I usually like to keep instruction use to the basics which makes it easier to implement similar programs on other platforms).
Mitsubishi manuals can be difficult to understand; most of the information is there but not always where you'd expect to find it and Japanese-English translations are not always very easy to comprehend.
I can understand why Peter had a problem with tech support; I guess a lot of the in-depth knowledge on the Mitsi is in Japanese. I doubt if all the technical info on the product has been successfully translated into English.
My other gripe about Mitsubishi is the fragility of its relay output cards, particularly when driving inductive loads; as long as you know this and use interface relays then it's not a problem.
I prefer the speed of on-line editing with a Mitsubishi; I'm not a big fan of the RS method which involves 4 steps instead of 2 with the Misubishi.
I have heard people disliking GX Developer but I have no axe to grind any more than with RS Logix.
I have not come across any reliability issues with either platform (Misubishi A series and AB SLC 500) I don't think the new platforms (Q series and Control Logix) have been around long enough to make a judgement call.
I agree with Parky about hardware and software "updates", I think AB have more guilty of this than Mitsubishi in the past.
I would guess that there are far more 3rd party products that will readily interface with AB, particularly in the USA.
If I were forced to standardise on one or the other, I'd probably go for Mitsubishi, mainly because of price but also, in a weird sadistic kind of way I've grown to prefer them over AB.