By perusing the internet and having a look around specific sites, I've collated a bunch of stuff about these upcoming behemoths that might be of some interest. I'll start with the Galaxy Note 5, probably the more compelling device for most people.
Galaxy Note 5
Starting with the Note 5, the first thing I will "note" is the curve on the back. Leaving the front of the device flat, so as not to tread on the toes of the S6+ (plus it would be difficult to use a pen on the curved display) the Note curves the back in to improve ergonomics. Using the new design language of the S6, a 5.7" device would be difficult to hold, so to mitigate this Samsung have given the device a more ergonomic back plate. Understandable.
Second is the SIM tray at the top of the device, being joined by the microphone but not an IR port. The S6 has an IR blaster, seems odd Samsung would choose to leave one out from the Note 5. I would say here that, judging by the size of the SIM tray, they could be using it double for the SIM and an SD card!
The clicky buttons from the S6 are intact - wonderful. They're the best buttons I've used on a phone.
The headphone jack is moved around to the bottom of the device instead of the top from last year. This is welcome from anyone who uses headphones with their phone in their pocket and just makes sense, it's stupid to put the headphone jack in the top considering the way around you use these devices and their size. If it were in the top any decently thick cable would overbalance the device to the top and send your pricey gadget to the floor.
Battery is obviously not removable, although, as I said above, an SD card is a possibility, HTC One M8 style.
Stylus has a new squared off and chamfered chrome bottom, likely very similar to the Note 4 stylus but I expect some difference in finish and build quality, to match the rest of the device. All metal, for example.
Technically, I expect to see the same or a slight iteration on the excellent chip in the S6 - the Exynos 7420. Maybe a 7422, slightly higher clocks could be possible but not needed. I can see Samsung adding another gig of RAM to get it up to 4GB and negating some of the TouchWizz RAM hog issues seen on the S6 - although these are FUD as it's allocated RAM we are seeing, not RAM actually in use, but public perception is king. Resolution changes are unlikely from the Note 4, as it was already class leading and beautiful as it was. I'd be happy with the exact same screen again.
Camera - staying the same - using the epic Samsung ISOCELL or Sony IMX240 as with the S6, both produce magnificent shots. I see Samsung possibly going all in house again as they did with the processor but it's unlikely as I don't think they have their sensor fabricating plants fully up to speed, hence why they used mostly the Sony sensor in the S6. OIS as well, obviously.
There's a chance they could go for the 21.5mp sensor also used in the Moto X Style and Play, the IMX230, but this would sacrifice pixel size on the sensor for detail, something which they already have enough of. Currently the IMX240 has 1.2 micron pixels, and the IMX230 has 1.12 micron and a larger die size overall. I think Samsung will stick with the 16mp sensor they already know how to get good photo sauce from. Why change what isn't broken.
One thing I do see Samsung doing, though, is going even more aggressive on the lense aperture to battle LG. F1.8 is a distinct possibility, and even F1.7 just to outdo their local rival. Either way, the photos from this handset will be very, very good.
The S6 Plus seems to be a lot less forward about revealing itself. At first it was difficult to tell if this was the new handset or just a render used from the S6. So I took about investigating.
I looked at the relative positioning of the buttons, the cameras, speaker grill and sensors. On the Spigen images this didn't lead to much. Both speaker grilles have 15 holes, for example. Then I came across this image as well, from Ringke as with the Note 5 images.
The buttons here are a lot smaller and occupy very slightly different positions on the sides of the handset. The capacitive buttons are differently spaced. The camera is visibly smaller compared to the rest of the phone too.
The camera size is more obvious here too. And again, no IR blaster either. Plus it looks like the bands for the antennae are chromed here too. Flat back on this model although the front is curved. Mic only up top.
Technically, again I expect to see 4GB RAM, 1440p display, maybe an Exynos 7422 but more likely the 7420 in this model as it's keeping the "S6" moniker.
Camera - as above with the Note 5 - sticking with the 16mp sensor they already know well.
There's not a lot else to say at this point about the handsets except possible dates. I've been looking at ordering times on Amazon, and most are already available or have mid-August dates. I've ordered one of each case, and we'll see what we can find out.
The last thing I'll say is don't expect them on the date shown on the screens in the renders - the August 25 date is followed by "MON" meaning Monday - that was August 25th last year.