[crunch]

Either way will not affect Fps adversely at all as long as you delete unseen polys. The thing is to limit the open areas, where the engine has to render a lot of data at once. I personally build my terrain in patches, adjust them , and then snap them together.
Now there is a seamless terrain with no overlap (that is where it could slow down--when the engine is drawing layers of terrain instead of 1 solid piece).
See the thread "low FPS outdoors" at http://www.dafront.com/~nemesis. Vonderbaklufts answers will surely explain FPS in outdoor/terrain areas. And how to insert vis_leafgroups to limit what the engine will draw, thus increasing FPS.