I try to decide which one I should buy. I know that the FSDT version is quite old, but Aerosoft doesn't have the greatest reputation regarding the frame rates of their sceneries.

I would be thankful for input on that matter.

FSDT has a demo. AS does not.

Try the FSDT version.

FSDT one was pretty good but is a little dated now. AS one is newer, and the pro version being built for p3d v4.  Also the groundwork and taxi layout is more inline from the charts today from what I recall relative to FSDT. AS seems to be a little hard on FPS at times.  As mentioned, try the free demo from FSDT 1st.  Its cheaper anyhow. 

I have owned both in all versions, I would say definitely the Aerosoft in the newer version for P3D V4. You can use the configuration manager to adjust the scenery which can help with performance.

Lee

I used FSDT until recently. I have finally switched to Aerosoft's one and I must say I prefer it over FSDT.
As the fellow simmers mentioned above, the FSDT version is old now, both as a product and as a rendition of the current LSZH airport.
Performance-wise I haven't noticed much difference.

thank you all! Your opinions are very helpful.