A New Hope...An Old Suit
Decide you must, which Vader you want to be.  With each movie, both large and small  changes were made to the suit.  For consistency, it looks best if one does their entire suit in one of the three schemes, rather than mixing and matching.

The biggest differences exist between the ANH Vader suit and the suit of the latter movies, so I've focused on the things that set the ANH suit apart for the other two.
Helmet:
The Under Chin triangle is smaller
The raised ridge on the helmet dome comes to a point between the eyes
The fiberglass material is a bit thicker on the dome flange
The helmet seems to ride lower on the mask (thus, the back of neck/top of eye sockets are more covered in the ANH version
Armor:
The shape is the same, but the paint job is a lot duller.  One can still see a two tone  gray / black paint scheme, but the armor is much less glossy, and the two-tone scheme is harder to see, especially with the inner cloak OVER the armor.
No cape hook 
(No cape hook for ESB version either)
Gloves:
The quilting on the gloves runs perpendicular to that of the suit
Inner Cloak:  Is worn on top of the chest armor
The Body Suit:
Only for real nit-pickers...The quilted leather has a much more crinkled look to it
Chest Box:
The box itself has a duller finsh
The coin slots do not light up
The blue button is of a more greenish hue and both that and  the red one have a duller finish
The rocker switches: One white one on the left..three gray ones
Side rods have no funny attachments on the tops and bottoms
There are no little buttons between the rocker switches or the red/bluish buttons
The is no writing beneath any of the coin slots
Belt buckle and boxes:
The boxes look like they are capped off with corresponding lids
The six do-hickeys light up (I think..need to check into that) and they are clear/white with reddish-orange rings around their bases
The green squares look more like a true green than an aqua/teal color
The horizontal portion on the botton  is more deep set and made from ??________?
(
not reinforcing hose)
Important ANH details
Back to: The Meditation Chamber
The Cod Piece: I think the cod piece is slightly bigger on the ANH version, and a little less refined...but not a tremendously noticeable difference.