Front / Rear - weight / aero?

Agree, that there is no such thing as need.  Marginal gains for serious outlay.

But if "want" translates to "get",  and this in turn gets you happy and out on it more often, then you will be faster.  QED cool bikes are faster bikes.  Deeper sections can be cool, but aesthetically go for shallower front if needs must, it don't look right the other way.  

Also, since modern 700c rims only weigh about 450g so when you buy lighter wheels you won't be saving much weight off here anyway.

It seems that the rotating weight myth refuses to die.

You lose more speed by lifting your head, taking your arm off the bars to wave or grab a drink, gel or bar. 99.9% of riders would gain far more by working on their pedal stroke and breathing technique.

The wheels don’t meaningfully slow down in the fraction of a second between pedal strokes. Think about it; assuming no slippage between the tyre and the road surface, the RPM of the wheel is directly related to your forward speed. If the RPM of the wheel was changing meaningfully in the split second that you ease off the power in a pedal stroke, then your forward speed would also vary. You’d be surging forward like a learner driver on the clutch.

Also, consider what happens when you freewheel. Your body, which weighs many, many times the weight of the whole bike, yet alone the wheels, has significant forward momentum. In that fraction of a second where your power drops during the pedal stroke, the forward momentum of *you* keeps the bike moving forward, and therefore the wheels turning, assuming again that there is no slippage between the tyre and the road.

Manufactures don’t play on rotating weight not because they can’t measure it, but because they would rather let the myth continue, than shoot themselves in the foot by debunking it.

The idea that rotational weight is un-important is clearly ignored by every wheel manufacturer ever, for very good reason. When it comes to simply the weight of the wheel at constant speed over a given gradient, then weight is *almost* immaterial. But that's not how we ride, as every pedal stroke (particularly on a steep gradient) accelerates the wheel so adds to the energy 'drain' on the cyclist.

The answer to the OP's question is very dependent on their riding. Are they road racing, criterium racing, time trialling, hill climbing etc etc. 

Saving 400gm on a wheel, if at the rim, will have an enormous difference on most riding done at the higher end of a cyclists power output. In a bunch, the aero has almost no effect at all, but due to the surges and accelerations, the weight will be important. 

So, the answer is.. it depends!

Wheel weight makes negligible difference. Removing 400g from wheels would save just 17 seconds in a 4 hour ride with 1200m of ascent. http://road.cc/171932

If you're chasing aero watts then upgrading the front wheel has greater impact than the rear. But a rider's clothing and position usually have a greater influence on drag - and therefore speed - than the wheel.

I agree with Gizmo_ about the squidgy bit between your ears, it's where the biggest gains can be made.

Most effective for what is a good question.  If like many, for you it means faster for the same effort then the answer is aero not weight, although be prepared for the gains to be marginal. 

The greater the depth of the section the better the aero performance, broadly speaking.  Deeper sections handle less well in cross winds hence some systems e.g. Enve having shallower section fronts, although by rights the front should have more impact as it is in cleaner air. 

The compromise I came up with was carbon tubs from Far Sports, 38 front, 50 rear coming out at 1285g weight and £430 plus tyres.  I believe they are marginally faster than my RS80 C24s but I sure can't prove it.

Most effective for what? Unless you are racing the only thing that matters in my opinion is how strong a set of wheels are and how much maintance they need. For practicl needs something like fulcrum racing 5s or mavic krysium equipes are good enough for most people will ever need.

Or if you want something that you like the look of just set a budget and go for the blingest wheels you can afford!

"In real world terms for an amateur" get ones that look cool (coming from the new owner of 55mm wheels that don't make any noticeable difference...)

Have a read of this:

Part 1:

http://alex-cycle.blogspot.co.uk/2013/02/the-sum-of-parts.html

Part 2:

http://alex-cycle.blogspot.co.uk/2014/12/the-sum-of-parts-ii.html

The math is beyond me, but this is one of the few sites I've come across that actually references a (seemingly) reputable source for their numbers. The formulas used come from a paper published in the Journal of Applied Biomechanics.

Short answer; rotational weight has such a small impact it's insignificant; overall weight matters a little, but it doesn’t matter whether it’s on the wheels or elsewhere; aero dominates on flat or flat-ish ground.