The primary difference is that the WIC-1DSU-56K4 only supports up to 64K and is designed for 56K DDS operation or 56K/64K DSU/CSU operation whilst the WIC-1DSU-T1 is designed to support fractional T1 or T1 clear channel operation.
If you only need a 56K / 64K circuit you could use either, if you need faster than this then you would need to use the WIC-1DSU-T1