Durham Nat Sci is a fairly new course - until about 10 years ago it was very small and it appears to have grown in response for falling demand for science (and to tap into high quality cambridge rejects).
Comparing the two courses:
https://unistats.direct.gov.uk/Subjects/Overview/10003270FT-F640/ReturnTo/Search v
https://unistats.direct.gov.uk/Subjects/Overview/10007143FT-FGC0/ReturnTo/SearchDurham is larger - but given the fact that this encompasses a lot of pathways you might still benefit from small class teaching. The Imperial class size doesn't seem to top 30 students (whereas for Durham you would be in a group of 125 students).
Durham seem to target people with 4+ A levels whereas Imperial seem more welcoming of applicants who only have the option to take 3 A levels.
Durham contact time is lower (34% v 41% at Imperial - with Durham at 32% in 2nd and 3rd yrs against 45-46% from Imperial for those years - Imperial only tails off in contact time in 4th year (presumably because of research projects which have non timetabled contact hours).
Durham has a much higher focus on exam results (>80% in yrs 1-3 compared to 69% and 75% in 1 and 2 at Imperial then less that 50% in the final 2 years).
Durham awards many more 2iis than Imperial (13% of completing students got a 2ii - everyone at Imperial allowed to finish 4 years got a 1st or 2i).
Both have very low drop out levels after 1st year - but at Imperial the level is 0%, at Durham 6% aren't still on the course after a year.
Imperial's cheapest accommodation is £1,000pa less than Durham's cheapest - and their most expensive is £1,000pa more.
Both have good student satisfaction results - but while at Durham there are some issues over feedback (promptness, detail and helpfulness) Imperial has a perfect scorecard:
https://unistats.direct.gov.uk/subjects/satisfaction/10003270FT-F640/ReturnTo/SearchAccreditation at Durham covers Chem or Physics - if you take the right modules.
Employment wise a LOT of graduates from both courses go on to further study - it might be useful to contact both universities and ask them more about what and where their graduates go on to. Most go on to "graduate" jobs - Durham is heavier on the IT job market, Imperial on Business. 5% of Durham NatSci grads go on directly to science employment, 20% of Imperial grads who are working have gone straight into science jobs.
If you like exams, want to work for IBM, can afford £7k for your first year accommodation, are taking 4+ A levels (and expecting As or above) and are happy that you can study independently and wont drop out then Durham is your best option.