Tom,
The two of them are really good. Fieldskit is mor specific about interlacing (it got de-interlacer, pull down remover and re-interlacer) and give you some options that you don’t get with Nattress (like making a ful Frame from each field). The Nattress is great and come with many interesting things in the same package.
You can make a try as well with “Smart de-interlace” of TooMuchTooSoon. Is free. Works very well if you set the “Motion sensitiviti” to the top.
Cheers,
Rafael