If you have the Surface Pro 2, it is not worth it upgrading to the Pro 3.
- The CPU is the same. (or Y series, as it apparently support connected standby, and I think only the Y series supports it, so it will be a slower CPU)
- The higher resolution means you'll need to use high-DPI settings, which means that most programs will appear blurry as they don't support high DPI. I expect the major programs to support high-DPI by the time the Pro 4 comes out, which is also means new Intel CPU.
- The pen looks like its N-Trig, while it is amazing where you don't need to calibrate it, it only support 256-pressure compared to 1024, and no programs supports it for pressure sensitivity tracking, beside Office, and Windows 8 Apps. Photoshop will support Windows 8 Ink APIs which will make it support. Oh, and you need a battery, and don't have the eraser feature, it's a side button I think.
The Pro 4 should be out soon, with the 5th generation CPU, I believe, or soon afterward.