You should clearly use ext file system as it is a native linux filesystem, so read/write access is faster than ntfs access (use a third party driver).
The only concern is that I don't think ext4 is supported right now by dd-wrt/tomato ? You can use ext3 without problem.
It seems that via optware you could install the ext4 module, but some report that it is buggy.
Ext3 is very stable, journalized, so use it! (more fragmentation than with ext4 but considerably less than ntfs).
WNR3500L running Toastman's Tomato.