I would prefer a portable DVD player.
A portable DVD player provides also the function of watch good movies.
You may also connect it to other display units like a big television or even a projector.
The extendability of portable DVD player is better.
However, a digital picture frame is more handy, and quicker in response.
If you are doing some roadshow to promote your products, a digital picture frame is more appealing and convenient.
Afterall, what matters is how you will use it.