DNGs are based upon the TIFF/EP6 Standard. So, DNGs are all TIFFs, but not all TIFFs are DNGs. TIFFs are always storing the image data as RGB elements. DNG has a little more flexibility in how it treats the image data block. RAW image data recored by the camera sensor is not RGB and needs to be de-mosaic'd and converted to RGB before processing engines like LR can work with the data. DNGs holding RAW data can keep the data as mosaic'd and non-linear. Or it be converted to RGB and stored as RGB. RGB data is not a RAW data structure. TIFFs on the other hand, Always store image data as RGB data.
Now, if you are scanning analog slides, the scanning software will most likely create RGB data. So in that case, it matters little whether your data has a TIFF wrapper or a DNG wrapper. All (almost all?) image processing software can handle the TIFF format. Some image processing software apps do not support DNG format or all flavors of DNG. For this reason it is better to save your scans as 16 bit compressed TIFFs.