Rome or London?

What is it you want to do when in these cities?
Rome can be expensive, but I believe that London is one of the most expensive cities in the world, though with the value of pound getting lower that may be changing.
Could you give us some more information?

We'd be doing the run of the mill tourist sites. Staying in hostels.

Carson,

I suspect the cost for seeing "run of the mill tourist sites" is going to be comparable in both locations. The Pound has decreased somewhat, but I believe the Euro has gone up?

I would probably weigh the decision more on which sights I wanted to see rather than the cost.

Happy travels!

Carson-
I wanted to go to London as well--but the EUR is lower compared to the Pound. Staying in hostels as seeing the low cost sites would be fine in London. In Rome-we found a B&B 10 minutes from the colloseum for $30 per person per night.

Rome, without question. Hostels and budgets accommodations in London can be sketchy. Food prices in London are extremely high as well. - The usual touristy things in London aren't too expensive actually, and so much of the city is in walking distance - but I don't think it's a good trade off for some of the bad housing experiences I've had on budget London Hotel stays.

Also remember England has the highest VAT tax in Europe are over 17%, on almost everything.
XE.com has the EU at 1.26USD and the GBP at 1.42USD - both great rates, but a slightly better deal in Rome...
If you do go with England, really check out the place(s) your going to stay first, and try and go in with a plan to eat cheap! Have a great trip!

Slight correction; VAT in the UK is 15% (temporarily dropped from 17.5%) and is actually one of the lowest in Europe. Even at 17.5% only Spain, Luxembourg and Cyprus had lower rates. For comparison the standard rate in Italy is 20%.