On 2015 vs. 2018, the performance for what the OP wants is going to be a huge difference. I have a 2017 Kaby Lake (high-end stock model, but no BTO upgrades), and I previously had a 2015 with the BTO processor that a friend who wanted the ports bought off of me. I was surprised how much faster the
2017 is in Lightroom - the 6-core 2018 is going to be in a different league, especially with the Vega as well.
At this point, I consider the USB-C ports an advantage - they certainly were not earlier in the lifecycle. An increasing number of devices are now USB-C - including the iPad, and if rumors are correct, soon the iPhone as well. Many newer cameras are USB-C - I'm presently charging my Nikon Z7 on a bus, using my MacBook Pro power adapter. Newer portable and desktop storage tends to be USB-C, because it's fast enough for anything except a few corner cases (RAIDed NVMe SSDs and 8+ drive RAIDs, especially with SSD caching, still need Thunderbolt). Most printers these days are wireless, and those that aren't are easily taken care of with a standard USB to USB-C cable
Better still, the Apple USB-C ports are also Thunderbolt 3, which allows them to adapt to any port ever made for a desktop computer - from
RS-232 and
Centronics parallel printer ports (USB-C) to
10 gigabit Ethernet and
Fibre Channel (Thunderbolt 3). Unfortunately, the 2-in-1 port adds cable confusion since the cables look identical except for subtle markings that can wear off, but aren't interchangeable.
I do wish they'd left one old-style USB port on there - the one real pain is flash drives. Some newer ones are double-ended, supporting both USB-C and classic USB - but the vast majority, and nearly all "giveaway" drives, aren't. The SD slot and HDMI ports are also noticeable losses. HDMI (and DisplayPort) to USB-C cables are commonplace, so monitors don't need an adapter - just a new cable - but the native HDMI port was useful for connecting to projectors and TVs using existing cables.
As for 2018 vs. 2019:
If the 2019 uses Sunny Cove, it's also likely to be late in the year - Intel is promising the mobile chips for second half 2019, which could easily mean "we got a few out the door for Ultrabooks on December 31". Maybe Intel will be faster than I expect, but their track record on 10nm chips leaves something to be desired... If the Sunny Cove release is delayed, the 2019 MBP could easily be WWDC
2020.
On the other hand, Sunny Cove is supposed to bring a meaningful improvement in instructions per clock - the first time in years we've seen a performance increase that isn't attributed to simply adding cores. I suppose at least some of the mobile chips could also be 8-core? If Sunny Cove is all that Intel says it is, it's a potentially major performance improvement,. It will certainly support 32 GB of RAM without the power compromise in the 2018 MBP, and it could support 64 GB.
There is a very slight possibility Apple might adopt mobile Ryzen, but it would take a completely different chip than AMD has now. Right now, there's no Ryzen even
close to requirements for the 15" MBP - all mobile chips are dual or quad-core and none benchmark as fast as the
2015 MBP - they are all 15 watt Ultrabook chips, not 45 watt workstation chips.
I would be
shocked if an Apple A-series chip showed up in a 15" MBP, at least in the near future. I would not be at all surprised if an A-series chip showed up in the
MacBook fairly soon, but the 15" MBP needs a much higher power chip that Apple doesn't have. The MacBook can literally use an iPad Pro chip, maybe with extra RAM, a higher clock speed and/or extra graphics cores. The 15" MacBook Pro would need not only a different chip, but a chip with different CPU cores - more Tempest cores wouldn't be enough. Apple's not going to make that investment in a high-power core until they've had other Macs on A-series for a while.
7nm (AMD and TSMC count such things differently from Intel, so it's actually similar to Intel's 10nm process) mobile Vega or Navi GPUs are also at least possible for the 2019 MBP. Depending on exact configurations, they could improve performance significantly over even the Vega 20, and certainly over anything found on previous MBPs. They won't go to a NVidia GPU, simply because they're Apple.
The AMD GPUs perform far better in the creative applications Apple cares about than they do in games. One of the ways Apple gets their stability improvement over Windows is that they
quite simply do not consider games at all when they design Macs. If games happen to work, great, but Apple won't lift a finger to make them work. Since games hit hardware really hard, and they tend to do unexpected things, ignoring them significantly increases stability.
The third possibility is a redesign - and the 2019 pretty much has to be either Sunny Cove, a redesign or both (possibly with the existing Vega 15 and 20, possibly with a 7 NM version). The 2018 is already using the final generation of 14nm Intel CPUs, and it's already had one GPU bump - so there's really nothing they can do without a new CPU that's not out yet or a redesign.
Nobody knows what a redesign might entail. They would probably replace the keyboard, but they're unlikely to move away from two basics that mean the new keyboard is not certain to be better. The most important is that the machine can't get thicker. Even if most of us would be fine with a couple of extra mm to get better key travel, Jony Ive isn't, and he's making the decision. Even more damaging, essentially the same keyboard has to fit in the whole line. The thickness of the keyboard isn't dictated by the 15" MBP - it's dictated by the
MacBook. Apple will probably eventually go to a completely flat keyboard (or a keyboard with key bumps that don't move) with a Touch Bar style display underneath and artificial haptic feedback - but that's unlikely in a 2019 machine. About the best keyboard we can hope for is a further refinement of the butterfly design.
I'd be a little surprised if the 15" got much thinner - they're already right on the edge of what they can cool, and the 15" has consistently used high-power processors and medium-power discrete GPUs. They won't add ports, simply because Apple never moves what they'd consider to be "backwards", and there's no reason to subtract any, with the possible exception of the headphone jack.
One possible positive redesign is a larger screen with more resolution, by reducing or eliminating bezels. Most of their competition is already 4K. If they could get the panels, I wouldn't be surprised to see them go to a 5K "Super Retina" design.
