ASUS ROG Strix Z270 "X"-motherboards: which one to chose?

The ROG Z270G is micro atx form factor, the Z270I is ITX, the Z270F and Z270E are full ATX. The Z270F and Z270E are the same exact board except the Z270F does not have a wireless card whereas the Z270E does. I have the Z270F myself with a 7700k and 3200mhz CL14 ram, works great no issues at all. However I believe all of those boards are well over $120.

The 3770k is still a very powerful processor, depending on what you do with your pc you may not even notice the difference between a 3770k and a 7700k.

Your current i7-3770 (non-K) is still a very powerful and capable CPU to pair with your MSI GTX 1080 Gaming X (https://docs.google.com/spreadsheets/d/17VRKPjyiTBx9Ewc2xkmaMZD2tA3gSOG3rNtH4OEiz3g/edit#gid=0). On that note, you don't actually need to upgrade your CPU (and your MB/RAM) just yet. However, going from an i7-3770 to an i7-7700K would have about 30% to 40% gain in performance (http://cpu.userbenchmark.com/Compare/Intel-Core-i7-7700K-vs-Intel-Core-i7-3770/3647vs1979).

But to answer your question directly, should you choose to get a Z270 motherboard from Asus, the main differences between the Asus' Z270-"x" (i.e., Z270-A, Z270-E, Z270-F, Z270-G, Z270-H, Z270-I, Z270-K, Z270-P) motherboards are the following:

Form Factor (Motherboard Size):
A, E, and F are standard ATX (305mm x 244mm) | H is slightly narrow ATX (305mm x 234mm) | K and P are narrow ATX (305mm x 226mm) | G is standard micro-ATX (244mm x 244mm) | I is standard mini-ITX (170mm x 170mm)

Memory (Number of RAM Slots and Max. Capacity):
A, E, F, G, H, K, and P have 4x DIMM slots at 64GB max. cap. | I has 2x DIMM slots at 32GB max. cap.

PCIe3.0 x16 slots (Quantity and Supported Modes/Speeds):
A, E, and F have 3x slots (at x16/x0/x4 or x8/x8/x4) | H has 3x slots (at x16/x0/x2 or x8/x8/x2) | G has 2x slots (at x16/x0 or x8/x8) | K and P has 2x slots (at x16/x4) | I has 1x slot (x16)

SLI and CrossFire (Multi-GPU Support):
A, E, and F support 3-Way CrossFire, 2-Way CrossFire or 2-Way SLI | G and H support 2-Way CrossFire or 2-Way SLI | K and P support 2-Way CrossFire only | I does not support multi-GPUs

PCIe3.0 x1 and PCI slots (Quantity):
A, E, F, and P have 4x PCIe3.0 x1 slots | H has 3x PCIe3.0 x1 slots | G has 2x PCIe3.0 x1 slots | K has 2x PCIe3.0 x1 slots and 2x PCI slots | I has none

SATA Storage Connectors (Quantity):
A, E, F, G, H, and K have 6x SATAIII ports | I and P have 4x SATAIII ports

M.2 Storage Connectors (Quantity):
All have 2x M.2 sockets

SATA Ports Orientation in Motherboard (for better cable routing):
A, E, F, and G have all (6 out of 6) SATA ports oriented 90-deg. | H has 4 out of 6 SATA ports oriented 90-deg. | K has 2 out of 6 SATA ports oriented 90-deg. | I and P does not have any of its SATA ports oriented 90-deg.

Wireless Connectivity (Bluetooth and Wi-Fi):
E, G, and I have Wi-Fi and Bluetooth on-board | A, F, H, K, and P does not have wireless connectivity

LAN Controller:
A, E, F, G, H, I, and K use Intel I219V LAN Controller | P uses Realtek RTL8111H LAN Controller

Audio Codec and Number of Audio Jacks at Rear Panel:
A, E, F, G, H, and I use ALC S1220A with 5x jacks | K and P use ALC887 with 3x jacks

Rear Panel USB Ports (Quantity and Types):
A, E, and F: 6x Total = 0x USB2.0 + 4x USB3.0 (Type-A) + 0x USB3.0 (Type-C) + 1x USB3.1 (Type-A) + 1x USB3.1 (Type-C)
G: 8x Total = 2x USB2.0 + 4x USB3.0 (Type-A) + 0x USB3.0 (Type-C) + 1x USB3.1 (Type-A) + 1x USB3.1 (Type-C)
H: 8x Total = 2x USB2.0 + 4x USB3.0 (Type-A) + 0x USB3.0 (Type-C) + 2x USB3.1 (Type-A) + 0x USB3.1 (Type-C)
I: 8x Total = 4x USB2.0 + 3x USB3.0 (Type-A) + 1x USB3.0 (Type-C) + 0x USB3.1 (Type-A) + 0x USB3.1 (Type-C)
K: 7x Total = 2x USB2.0 + 2x USB3.0 (Type-A) + 1x USB3.0 (Type-C) + 2x USB3.1 (Type-A) + 0x USB3.1 (Type-C)
P: 6x Total = 2x USB2.0 + 4x USB3.0 (Type-A) + 0x USB3.0 (Type-C) + 0x USB3.1 (Type-A) + 0x USB3.1 (Type-C)

Internal USB Headers (Quantity and Types):
A and F: 3x USB2.0 (9-Pin) Headers + 1x USB3.0 (19-Pin) Header + 0x USB3.1 Header
E: 3x USB2.0 (9-Pin) Headers + 1x USB3.0 (19-Pin) Header + 1x USB3.1 Header
G: 2x USB2.0 (9-Pin) Headers + 1x USB3.0 (19-Pin) Header + 1x USB3.1 Header
H: 2x USB2.0 (9-Pin) Headers + 2x USB3.0 (19-Pin) Header + 0x USB3.1 Header
I: 0x USB2.0 (9-Pin) Headers + 1x USB3.0 (19-Pin) Header + 1x USB3.1 Header
K and P: 2x USB2.0 (9-Pin) Headers + 2x USB3.0 (19-Pin) Header + 0x USB3.1 Header

Fan/Pump Headers (Total):
A, E, and F have 6x | G and H have 5x | K has 4x | I and P have 3x

Power Phases (for Overclocking Voltage Regulation):
A, E, F, and G have 10 | I has 8 | K and P have 7 | H has 6

Costs (as of today):
P costs $108 (Newegg) | K costs $140 (Amazon) | A costs $155 (Newegg) | H costs $160 (Amazon) | F costs $174 (Amazon) | I costs $180 (Newegg) | G costs $190 (Amazon) | E costs $200 (B&H)