According to the Shulchan Aruch, Torahs should be written on Gevil, Mezuzahs on Duchsustos and Tefillin on Klaf. This is the ideal, but all three of these things could be written on Klaf if necessary. Basically Klaf is kind of an all purpose material. For whatever reason, most Ashkenazi communities have written all of their Torahs on Klaf for centuries, even though they admit that Gevil is theoretically better. But most Sephardic communities were still using Gevil up until a generation or two ago. When most Sephardic Jews were expelled from their various Muslim countries, they mostly started using Klaf.
A grass-roots movement has started in Israel to bring back the Gevil for Torahs. The people behind it are trying to convince all Jews to write their Torahs on Gevil, the ideal way. It is quickly taking root amongst the Sephardic Jews in Israel. I suspect that it will meet with a lot of resistance amongst Ashkenazi Jews. Many Sephardic Jews still remember the Torahs that their parents brought back from the "old country" which were written on Gevil. The Ashkenazim have no such memories, so it seems somewhat foreign to them.
The other day a man visited my shop who sells Megillahs and other items written on Gevil. This was my first time seeing and touching an item written on Gevil which wasn't hundreds of years old. At first it seemed very strange to me. The Gevil is much thicker than Klaf. It almost feels like very good leather. It is also much darker than Klaf. But I must say that it felt very special. I can't put my finger on it but it just felt right. Klaf is very dry and brittle. If you fold it, it will crease and the letters will crack. The Gevil is supple and doesn't crease.
In the mean time, I will wait and see how this old/new trend goes. If a few years from now, you start to see brown Torah scrolls in your Synagogue written on Gevil, remember you heard about it here first. Here is a link to the site of one group who is bringing back Gevil. http://www.globaljms.co.il/