Samba (or any other tool that works on files) shouldn't care if the shared directory belongs to filesystem on a partition, directly on a device, on a remote machine; or exists within regular file on disk or in memory; or even if it spans over multiple filesystems because of mountpoints somewhere within the directory.
(Note: in general some tools may be set to avoid descending to another filesystems, it is irrelevant to your question though.)
The rule is: if you are able to mount the filesystem with the right permissions then Samba should work with it. It is the OS job to worry how the things work under the hood, not Samba's.
any performance/stability difference?
In your case I expect no difference because having a filesystem directly on
/dev/sdb is nothing fancy. In fact this will bypass one level of abstraction so in theory you may even improve performance, yet I guess this gain will be negligible.
The disadvantages I can think of right now are:
Future problems with entities that expect to have a partition table on the device. Samba definitely is not one of these entities. You and other admins (if any) are. In two years your partitionless setup may cause "WTF?" in somebody's head.
Future troubles in case you need to do the thing for which we have partitions in a first place: logically splitting one block device to several. When you decide you need two partitions, it will be fairly easy to shrink the existing one and make room for the other... if there is the existing one! Partitionless setup will require more work, more knowledge and insight, and will give you more occasions to mess it up.
Now imagine the two above combined. With this image in mind I advise you to go with
/dev/sdb1 because it is the least surprising thing.