The 20m mesh works 100% with FSX (I updated the mesh product page a couple of days ago, you may want to check it out
www.redbarent.com).
The topo is a bit more complicated:
1) FSX ships with very good data, although there are some flaws
2) FSX is not 100% backwards compatible in regards to the topo, I need to still create some instructions on how to make it work
If you're planning to use FSX only, get the 20m mesh. If you're also planning to use FS2004 get both. That's what I would recommend.
Christian