There has been some really good advice on here.
I'd like to take a few lines to clarify some details about paddles and hopefully help some people out in the future.
A wing/cupped/scooped blades is for fast boats - they are designed to optimise the forward stroke and improve efficiency's in this regard by 5 - 10% over a std flat blade. They are horrid in a slow boat. They are not designed to optimise any other stroke and are difficult to use for anything other than forward paddling. To gain this efficiency they need to be relatively rigid - they should never be made in plastic as it down grades the efficiency and still leaves you with a hard to use paddle (lose lose).
It is worth noting that they are harder to use in a forward stroke as well - their instability in the water unless used with precision can degrade their efficiency.
So in short only use a wing paddle if - your boat is quite fast, your keen to study the art of paddling to a relatively high standard and your prepared to buy a decent quality item.
For everyone else:
A flat blade or dihedral blade (one that slopes away at the top and bottom) is easy to use, stabile, easy to be quite efficient and capable of a wide range of strokes.
These tend to come in 2 styles, a shorter wider blade - suited to high angle paddling, or a longer thinner blade suited to low angle paddling.
This angle is the angle of the shaft relative to the water during your forward stroke. top hand above shoulder height is high angle, top hand below shoulder height is low angle.
Most rec paddlers are low angle style - but use a high angle blade... It still works as does a plastic wing paddle, but its not ideal.
Then what makes a large difference and what the original question is asking is the manufacture of the paddle.
Typically a cheap paddle will have an alloy shaft and plastic blades. This results in a heavy unit with flexible blades and a rigid shaft. The blades flex losing your grip on the water, the rigid shaft gives a higher load transfer to your tendons and joints etc.
Typically a premium paddle will have a composite shaft and blades (composite being a mixture of resins and a material such as carbon, kevlar or glass), this results in a lighter weight unit with rigid blades and a flexible shaft. the blades catch your waters and the flexible shaft reduces load on the body.
Between these two points you can improve 1 or 2 of the 3 traits at a time which creates your paddles price range generally $60 - $400.
Now within that there are a variety of methods of construction both within plastic blades and composite blades which is too in depth to dictate here but if people are keen to learn more feel free to email me.
Blade angle - offset. 45 - 60 degrees is the most common. 60 degrees is classically viewed as the ideal angle to reduce windage on the top blade without overflexing the wrists yet flexing them enough to promote healthy body rotation through your stroke. 45 degrees is a little more relaxed stroke - not quite as classically perfect - but then again neither is most peoples forward stroke technique - often favored by low angle paddlers.
Paddle length - how long is your piece of string? Again there is no perfect in imperfect answer to this question, it is effected by the conditions you paddle in, the type of craft your paddling, the distances you paddle as well as your height and strength. So there is no generalised correct length - which is why cheaper paddles tend to be in set sizes and more specialised paddles tend to be custom made to suit the user.
multi piece, adjustable, bent or crank shafts or your std 1 pc shaft can also make a difference and are worth looking into when you consult a professional about your purchase.
Most guys on here will look to paddle 215 - 220cm in length but overall will vary between 210 and 240cm.
Horizon Line Canoes