Money is always an issue or it wouldn't be "scarce." As you note, you don't want to waste it.
So it comes down to need. Do you need cellular or GPS? Think about if it would come in handy in your current routines -- daily, weekly, monthly, whenever. Then you can figure out if its worth the extra $.
If you can tether and have an iPhone then I'm not sure I see the value of the upgrade. I have cellular model myself but I can't tether b/c I have a grandfathered data plan which doesn't allow tethering.
The only caveat I can see is if you travel between London and the US. Then, with the new Apple SIM it might be useful because you can switch carriers depending on where you are w/ local data rates, not "rape me" international roaming ones. (EE and TMob).