I also received an offer from both PwC and KPMG (I went for PwC in the end), but truthfully I think the decision is a personal one.
They are both great firms and have great opportunities, and look strong on your CV. Cultures and work/life balance are similar. If you are doing Audit, you are going to have a period of busy season (roughly Jan-March) where the hours will be terrible. Then they will ease off for the rest of the year and be fairly flexible. Who did you feel more comfortable and at home with in the interview? People don’t think about this usually, but its important!
Have a look at the areas you would be joining in each firm - what clients do they have? Which clients would you like to work for?
When I joined, PwC had the vast majority of the FTSE100 audit clients, however since audit rotation, things are changing up so have a look at who you may be working on!
Also, do you have an idea about what you want to do after audit? If you want to do something else or move around the firm, have a look at other areas in both PwC and KPMG. I moved in to transaction services/ corporate finance and at the time PwC were stronger for this.
Hope this helps! Good luck.