What should I use to finish my plywood table top if I intend to leave the end-grain visible for a modern look?
The plies (layers) in plywood are run in alternating directions, so approximately half of the layers visible on any edge will be end grain and half will be the long grain.  End grain usually absorbs more finish which makes it look darker.  If you want to minimize the difference between the plies you could seal the edges of the plywood with shellac before you apply a top finish coat over the whole piece.  Alternately, you could let the difference in the darkness of the plies be a 'design feature'.  

Other than the alternating plies / darkness issue, the rest of the finishing depends more on how you want the table to look and how it is intended to be used.