I don't have F1 2015, but that does sound very impressive as a weather system. I do think the FM6 puddles are impressive too, but only in small doses, as it were. The first time I did a wet race in FM6 I was blown away by the weather, but once I realised the puddles are the same shape/size and in the same location every race (plus the fact it never gets wetter or dryer) means a lot of the initial interest was eroded. I think the thing that makes a weather system really work is the fact that it's dynamic and unpredictable - it's not just about the simulation of the wet track itself. That's why I like the sound of the F1 system.
Real weather is dynamic but not entirely unpredictable. A track is always going to form rivers and puddles in the same places, because that's where the natural dips and hollows of the land are. You can see the weather coming in, and in real life you'd have someone on the radio helping you out, even if it's only a friend standing on top of the pit buildings watching the clouds.
It's a bit like learning where the bumps are on a track, once you know where they are then that's solid knowledge regardless of whether you're driving a car that's strongly affected or not. With weather, there's going to be some variation based on exactly how much water there is, but you're going to know which parts of the track to avoid and which to take care on. Because really, if it was fully unpredictable it would be impossible. You can't do anything if you start hydroplaning mid-corner when you're at ten tenths. You'd be into a tree and dead.
I think what makes dynamic weather interesting is that it requires adaptability and additional skills from racing in the dry.
Firstly, one needs to learn the new lines and cornering speeds suitable for wet conditions. This means learning where the puddles and rivers are and how to avoid them at maximum speed. Some areas that might have previously been passing opportunities may now not be, because of standing water. Arguably it also becomes much more important to drive smoothly.
Secondly, one has to be able to judge the extent of the rain. Being about to do it visually is important, at least as an approximation. This is a major failing of the GT5/6 system, and why they had to implement the rain gauge. One should also be able to feel how the car is behaving, and refine the estimate of the available grip and level of water accordingly.
Thirdly, one needs to be able to adapt to the ongoing changes. If the rain gets heavier on your windscreen, that means in a lap or so there's going to be a lot more water on the track. Push hard now while you've still got grip, then take extreme care.
It's another variable to pay attention to, and what separates the men from the boys is those that can see the signs and adapt appropriately before the conditions catch them out.
This is why I don't really have much of a problem with FM6's weather. It's extremely limited, but seems like a decent simulation of driving in a steady downpour. That's only one of a million different possible situations, but it's something. It fits the T10 philosophy that I was talking about before, a limited feature that is of extremely high quality within it's scope.
GT6's weather isn't great either, because while it's "dynamic" most of the skills that you would use in the real world to be fast don't apply. There's no puddles or rivers. You can't see the water on the track, you have to look at your water gauge. The way the tyres respond to water on the track is iffy at best, slicks are mostly fine instead of a death sentence. That also is consistent with the Polyphony philosophy.
To me, it seems like they're both not great and it's swings and roundabouts. One gives you a fairly excellent simulation of one specific situation, the other gives you a fairly average simulation of a range of situations. But that's just my take on it.