Main differences:
The 1500 is the big brother. Think of the 1500 as the ControlLogix and the 1200 as the CompactLogix (although it actually probably overlaps with micro800 and a small compactlogix for memory). They are largely the same, although the 1500 does have some extra functions. 1500 has more work memory (RAM) and faster speeds. It has built in HW diagnostics, and PLC based alarming features. Both can be safety PLCs. Both have Profinet capabilities built in, and programming will be done over Ethernet.
They are programmed largely identically. 1200 code can almost always be copy/pasted into a 1500, and 1500 code can usually be copypasted back. 1500 can do everything the 1200 can, but the 1500 does have extra instructions that the 1200 doesn't. 1200 does not have S7-graph or STL. 1200 also doesn't have some legacy instructions that exist in the 1500 for S5/S7-300 migration purposes.
Most new features in the 1500 trickle down to the 1200 a few versions later. 1500 got the option to be an OPC UA server around V14, 1200 got it V16.
Motion is handled a little differently. 1500 motion is based around VFD/Servos whereas 1200 motion is more based around steppers. Still programmed the same, but the configuration options are different for some reason.
1200 is programmed with Step 7 Basic, whereas 1500 requires Step 7 Professional. Both are in TIA Portal and look and feel the same, but Pro costs 10x more. Both Basic and Professional include PLCSIM for simulating code without downloading it to a physical device.
1500 needs a memory card to store the project (Load Memory, think hard drive). 1200 has some Load Memory built in, but you can use memory cards to expand if needed. Both can use the memory cards for things like data logging to a csv file.
HW wise, 1500s come in a few different form factors. "Normal" 1500 is ET200MP, where the CPU comes with a built in display. ET200SP style CPUs come in the form factor of the interface module of slice IO. ET200PRO CPUs fit into an IP65 IO rack. They all program the same, although you occasionally run into an issue where some external tool won't work with different form factors (like PLCSIM). 1200's only come in HW flavor, although different 1200 CPUs allow for different numbers of IO modules to be connected.
Learning:
Most things you learn on one platform will apply to the other. If you're buying it with your own money, 1200's relatively cheap (couple hundred bucks, and a few hundred more for software). If the company is buying, I think the smallest 1500s are less than $1k (although the Pro software is a couple grand more), and you get to learn everything.
If you're building a serial machine, and you know a 1200 covers your functions, it can make sense to save a bit on every machine. If you're doing a custom project or you're doing R&D on a new machine and aren't sure, I'd always recommend a 1500, just so you don't have to waste time/money sizing up later.
