A picture field is a better choice if you need to see the preview of the first page. By default, it is stored in the field and can be displayed in an input form.
A Blob field proves to be a good choice if you do not need the preview and if you want to improve the speed of searches on the contents of the field. When using a Blob that stores a 4D Write document, there is no need for expanding the contents and storing them in a 4D Write area to perform a search on the 4D Write contents.
Also, your choice may depend on what processing you perform on the 4D Write contents. Taking a look beforehand at the 4D commands for Blobs and pictures should prove helpful.