Um...Kent is not a city, neither hertfordshire. They are counties.
I live in Essx which is in between the two and is probably the best of bost. Hertfordshire is landlocked, and from my experience of driving through it, is very boring, with ugly, un-inspiring towns built in the 50s such as stevenage and welwyn garden whatever it is. Kent is better, its got a coast, towns a bit boring. Essex has the longest and best coast undoubtedly, fewer large towns like kent but quite a bit to do. Also its very integrated with the east end, very easy to travel between the two and culturally, especially in southern/west essex, very similar, albeit very "white" compared to london.
Oh, I'd also say Kent has Bluewater, hertfordshire has no centre like that, essex also has lakeside which is literally 10 minutes away. Kent is connected to mainland europe to obviously