My 2 cents:
Render Layers are worth the price alone if you use TG as part of a pipeline and want to combine all sorts of assets in a compositing app later in the process.
Import - Export capabilities:
- FBX import (even though it is limited quite a bit)
- Terrain Export (even though it needs a lot of cleanup after export) Working in a 3D application viewport is a lot faster than trying to do Non-Terrain Work in TG. Placing objects, for example.
- EXR output for heightfields - take the EXR into a painting app and paint on it more precisely and faster than you can in TG. Import back into TG and you have a a fast turnaround without masking everything TG
- Camera data import-export can be useful
If you do almost everything inside of TG then it might not be worth to pay double the money (no idea how much the update costs).
If you do use some sort of pipeline jumping around between different software for different tasks then it is worth the money, I think.
Personally I find it easier to use different apps for different tasks.
A rather important factor for me to buy the Pro-Version back then was the slow development cycle TG has.
1 year maintanance gives you 1-2 updates. Major version changes take much longer. I only paid for an update once or twice, I think.
So, buying Pro you can be sure to not miss out on a lot of new features in the next 2-3 years without updating.
Right now I would advise against paying for an update to Pro - Just wait till Terragen-5 arrives. Might take some time, still, but there will be discounts on updates or full the version for sure.
Or wait until the first 5.point update arrives, chances are, that your 1 year maintenance gets you 1 or 2 major updates after that.
At the moment it looks like you do not really need the Pro-version to produce better pictures. Your learning curve is quite impressive
CHeers, Klaus