I have faced similar dilemmas. First and foremost, until the job is offered, there is no need for you to make this comparison, so each job should be considered on its own merits right now.
Make sure that you are comparing apples and apples.
Write up a list of the benefits of both location (and I mean physical benefits - such as their specifics of insurance coverage, vacation time, tuition reimbursement policy, support of professional development, salary, etc.). If necessary, after the job is offered, if they haven't yet provided this information, request that they send it to you for review before you make your decision.
Write up a side by side comparison of job functions and possible future expansion. Review this list to see if one job has more immediate satisfaction, more appealing responsibilities, etc. Then look to the long term. Which one has more potential for long term growth? Which one will be better for your resume (i.e. prestige factor, title difference, etc.)?
Then make up a list of intangible values, the things that will make your life better (or worse) (shorter commute, better liked industry, size of company versus your personal preference, co-worker's personalities, summer hours, likely boredom factor with the role, etc.).
When I was faced with this dilemma (offered a job at one company with a second interview pending from another company that week), I told the first offer that I wanted to complete interviewing, so I would be making an educated decision on the best fit for me and I asked for a week to consider. They agreed to my request and I ultimately ended up accepting their position.
Once you have these worked out, you will probably find that there are more checkmarks in favor of one position than another. However, before you make a final decision, especially if they are close, identify possible opportunities to sweeten the pot. If one job seems like your ideal except that they only offer xyz and you want abc, the negotiation phase is your time to explain that you are looking for abc and can they meet your need. What modifications you make to their offer, make sure you get them in writing. I think you are in the UK, so contracts are common there, so you might have done that anyway, but in the US written offer letters aren't as common as we might think.
Good luck with your decision, but I would just caution you again, until you have two offers, you DON'T have two offers and should consider the merits individually. Don't turn down one offer, expecting another to come through, since it may not materialize (ask me how I know!).
Marie
