Not directed to me but I don't think this will be the case.
Rather than going for the "newest" why not get the one that best suits your needs.
I have both models and owned the 12x50 about a year before I got the brand new version of 15x56. Which to I like best? It depends on what I'm doing. IMO, I think both are a struggle (or at least, not ideal) without a tripod so I rarely use either of them hand held. 90% of my hand held use over the past 6-8 years has been with stabilized Canon binoculars.
The 12x50's seem to be brighter at the beginning and end of the day and can peer into shadow areas a bit better. I like to use them when walking through the Ponderosa forests while toting a tripod. The FOV is sufficient enough that I usually don't carry another optic. If I do, it's something much smaller like the 8x25CL, which gives me more freedom of movement to find animals and them aim the 12x50s at them.
The newest 15x56's (I've owned all 3 main versions) were a big step up from version 1 & 2, in almost every area of concern. I owned versions 2 & 3 simultaneously and I was impressed with the improvements. Still, the FOV is limited, so I tend to use them in more open county and at long distance and I also tend to use them in conjunction with an 8x32.
I would say that deciding between these to binoculars has more to do with choosing between an optical geometry rather than a model because they're both at or near the top of their class for the two respective magnifications.
If you're going to do any hand held viewing, just go with the 12x50's. The 15x56's are a dedicated tripod binocular that's great for revealing a good bit more detail in what you're looking at if you don't mind restricting yourself to a tripod.