Shutter speed or ISO?

I think in this particular case, "photochoping" is not cheating, but making up for an improperly exposed image in the first place. You can do more work up front, or more work on the back end, but eventually, you end up doing both to varying degrees.

I'm not even sure what would be considered "cheating" in post-processing an image. Unless you mis-represeent a modified imagee as reality (people where they never were, etc.), it's just part of the process of photography.

The easiest thing to try would be to pop up the flash while in P or Tv mode (1/200 sec. tops) and see if it can provide enough fill for your shot. Alternatively, open up 1-2 stops (EC) since you're shooting a backlit subject, or move around so the sun is at your back when you take the picture.

True... I guess I did not think of it that way.

Or get a monster piece of white bristol board and reflect light into the chopper.
Now that I'd love to be a witness to.

Fill flash would be your best bet... but range may be an issue. Try it out, what do you have to lose? I'd use a speedlight though, something tells me you won't like the on camera flash.

As far as photoshoping being cheating... I have to say I disagree with you. Photoshop is a digital dark room.
Would you call Ansel Adams a cheater to his face because he used localized burning techniques in his non-digital prints? I wouldn't... he not only was a genius with a camera... but an artist with interpretation.

You could take the best photograph that's ever come off a camera and still improve it using Photoshop. It gives you the freedom to present your image in the way that you want... without having to smear vasoline around a filter or all of that other photography "cheating" as you call it.

I don't think changing the shutter speed or ISO would have given you a different result. It's a lighting issue... but only a minor one.
Another issue is that the image is a little flat. Even playing with your camera setting wouldn't correct that. With photoshop, you can restore the contrast that was once there.

That won't fix the exposure issue. An L lens isn't a cure-all.

I was aiming that comment at this comment:

It's a cure-all for high bank balances though... it'll fix that right up.
However, with the improved optical quality (... I'm stretching here...) you would be able to recover more shadow data while post processing... yeah, that's it.