These are my favourite zoos, and I agree with previous comments. Both are rural zoos of similar scale (Howletts 90 acres, Marwell 140 acres, Port Lympne 600 acres) centred by stately homes, set up in the mid-twentieth century with conservation as the catalyst and the welfare of animals rather than people in mind. Both keep rare and unique species, with ungulates been Marwell's speciality: All three zebra species, the hub of Okapi, Somali Wild Ass and Pygmy Hippo breeding in the UK, the only Sable Antelope, Dama and Dorcas Gazelles in the country along with the only Takin and Black Wildebeest bar one other collection, and also like Aspinall, success with the cat species. The big difference with Marwell as mentioned is that it has more 'themed' exhibits such as Roof of the World, Life in the Trees and the African Valley, with more set to come in the masterplan, the next being a new cheetah exhibit which will hopefully open this year.
That said, you can definately feel a similarity between these three parks (along with the Cotswold Wildlife Park IMO), and indeed John Knowles and John Aspinall got on very well together. In fact, when Howletts first opened to the public, the hunting dogs and many of the deer and antelope were on loan from Marwell. One of Marwell's last tiger cub litter went to Howletts, and is now the breeding female in their successful amur tiger group there.
I would strongly recommend all if you could manage it.