In the xylem, what is the difference between the vessel tubes and tracheids?

Tracheids and Vessel Members are the two fundamental types of tracheary elements that occur in the xylem.  In the mature state both are more or less elongated cells (some vessel members may be drum shaped), having lignified secondary walls and containing no protoplasts. They differ from each other in that the tracheids are imperforate cells having only pit-pairs on their common walls, whereas the vessel members are perforated in certain areas of union with other vessel members.  Thus, the vessel members are joined into long continuous tubes, the vessels.  Liquid moving through these structures passes freely from element to element through the perforations, whereas min the tracheids it traverses the walls, particularly the thin pit membranes.  Vessels have perforation plates which commonly occur on the end walls, but they may be present on the side walls too.  

All gymnosperms possess tracheids.  Tracheids are usually much longer than vessel members. The gymnosperm order Gnetales are the only Gymnosperms to possess vessels.  Not all angiosperms angiosperms have vessels.  They are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae.  
Some angiosperms have both tracheids & vessels, others have vessels only.  Members of the family Tiliaceae may have wood with tracheids or wood without tracheids.

they  both affect transpiration the same.
vessel tubes are more "advanced" . the holes are lined up between the different cells so water passing moves straight. (they form a sieve plate)
in tracheids the holes are not aligned.
and by the way, a plant has only one of the two types not both! so they both form the same function!
as i said, higher plants  (more advanced) use vessel elements (tubes)