Can a notary public act as a witness? 

We already have a very thorough blog entry entitled:
 
Can a notary be a witness?
 
That contains all pertinent details regarding notary witness requirements and procedures.  To sum it up, a notary can act as a witness in any state in their capacity as an individual.   However, in Delaware, and Washington State, and perhaps a few other states, a notary can do witnessing as an official notary act, and charge an official fee set by the state for their services.
 
Is it better to hire a notary to be a witness?
Since notaries work exclusively with signers, signatures, documents, foms, certificates, etc., many people feel that notaries are better equipt to handle the (not so vigorous) responsibilities associated with being a witness.  Additionally, notaries have been screened by their state and are more likely to be honest upstanding citizens — or at least that is what many of us like to think.  The reality is that some notaries have no idea what they are doing, while others are very particular about witnessing, and documenting information in conjunction with witnessing. 
 
Why would it matter who you picked as a witness?
If someone witnesses a document signing or a Will signing, it doesn’t matter much who they are provided they are at least 18 years of age.  On the other hand, if the witness ever needs to be contacted after the fact, it is good if the witness has lived in the same place for a long time so you know where to reach them.  If your witness works for the circus or lives in a caravan and moves around a lot, you might never see them again. Having a witness who is a notary might help if they provide some extra documentation for you. Additionally, a mobile notary supposedly knows how to show up at the appointed place at the appointed time and might be more reliable with logistics.

You might also like:

Types of witnesses in the Notary profession
http://blog.123notary.com/?p=5664

Credible Witnesses from A to Z
http://blog.123notary.com/?p=452