I have the standard version of A6 so I dont have access to the hmp part of Med. Having said that, I have fournd some very good editors, "Bryce, Geoscape, Nems Terrain Gen. and each has its benifits.

They say that MDL takes less time to compile, I like Nems Terrain Gen. It exports to the .map format. Now the compilation time is extended based on the size of your terrain but I find the benifits to be exceptional. Geoscape does directly export the the MDL format if you so choose and It is decent in its own right. It has good textureing mode according to the height of your land. Bryce is a top of the line editor with many benifits also.

It all depends on how detailed you wish to go, I'm sorry I cant say much about the HMP format, someday I will upgrade probably straight to the COM edition. The chunked terrain system allows for large terrain levels I've been told and the texturing mode in MED is supposed to be good.

anyway, I hope this helps you in your decision making, its the best Info I can provide.

Take care.