I've mainly used FSG freeware mesh for years and been pleased with it, but I can't compare it to anything else because I've almost never tried anything else. The single exception was early on, when I tried some mesh that I'm pretty sure (not certain) was by Raimondo Taburet, and it was incompatible with the corrected waters of FREEflow New England - I had lakes hanging on mountainsides at crazy angles. I can't say that FSG is perfect - it probably isn't (what is?) but at least my mountains are steep, located properly in relation to lakes, rivers and coastlines, and my lakes and rivers are flat.
As for having different levels of detail in different paces, definitely yes. You'll want the highest detail you can get in your local area, where you're familiar with every little hillock and vale, and in places with a lot of spectacular vertical scenery, like like the Grand Canyon, the Alps and the Himalayas. No surprise - these are just the sort of places that the highest resolution mesh is available for. (Well, except maybe your home town, depending on where you live.)
I read here (I can't recall who wrote it) that you can have mesh of different levels of detail installed and that FS will know enough to read and use the most detailed mesh it has in each location where you fly, so you can just drop all your mesh into one folder, and FS will sort it out. I followed that advice and it works for me; I see the extra detail in the places where I expect to see it. I keep my mesh folder very low in the scenery stack, with pretty much everything else layered on top of it. I don't know how necessary that is, but it seems to make sense.
Here's a little tidbit about high definition mesh. I've lived in this town for more than forty years and have explored it thoroughly on foot and bicycle. But when I installed high def mesh in FS9 and flew over it, I discovered a couple interesting geological features that I hadn't spotted in the real world, and when I went looking for them, sure enough, the real ground was just like the sim.
