How important is portability to you? Will you primarily use the machine as a desktop computer connected to an external display? If so, how often?
With me, for example, I’m connected to a couple of desktop displays about 90–95% of the time. However, that 5-10% when I’m not is when I’m running around here and there with my computer, so physical size becomes an issue. When I’m using it as a stand-alone machine, transporting it between jobs, taking it on a camping trip, or strapping it to my motorcycle, I need a computer with a display that’s big enough to comfortably use yet small enough to transport with minimal hassle. Add it all up, and the 14" Mac is perfect for me.
The perfect size for you is a different problem and one that only you can decide. If you’re using your machine without external monitors much of the time, I’d suggest the Mac with the larger monitor.
You mentioned UI/UX design, which doesn’t need the raw power that video or 3d work demands. Are you a gamer who needs an ultra-fast mac? I’m not a gamer, and I rarely do 3d or video editing, so if it were me, I’d probably choose the M1 Pro chip over the M1 Max. However, I haven’t thoroughly researched the difference.
As for memory, with the work you described, you can certainly get by with 16GB. That’s what I’m using right now, and it’s fine. However, I’d still likely increase the memory to 32GB on a new machine to help future-proof it for two or three years.
I don’t need a ton of storage in my computer since I only keep the essentials there and offload everything else to a removable drive, so I’d likely choose the 512GB model. Still, if you do things differently and don’t rely on an external drive, the 1TB is probably better.
Regarding cost, I’m on board with your thoughts. I spend nearly a third of my time tied to my computer, so even though I’m all for saving money, getting the machine I want and need trumps saving some cash. There are other computers cheaper than Macs, but if you don’t like the Windows OS and have a lot invested in workflow and the Mac ecosystem, saving a few hundred Euros/Pounds/Dollars/whatever is hardly a good bargain.