2G actually uses less power than 3G. But all phones use more power in poor signal than in strong signal - "little to no" sounds like the worst case, because dropping in and out of coverage really does use power.

Constantly swapping between 2G and 3G is a drain as well, so if you are somewhere with good 2G but patchy 3G you will save power by switching to 2G only mode.

 

But adding to this, if using the phone for any data tasks (browsing, email download) could it be argued that 2G is slower so tasks take longer, using some more power?

 

Yes, I expect that is true.

 

I wonder how negligible the difference is when using data between 2G and 3G...

 

Actually if you are browsing a slower download has the extra drawback that the screen is on longer - though of course if it's too frustrating you might stop browsing sooner!

Then there's the extra factor that since the phone adjusts its power level according to reception, the relative demands of 2G/3G have to depend at some level on the relative signal strengths.

But the bottom line on the OP's question is, if your reception is bad you definately expect the battery to run down faster, no matter what you are doing with it.