Hi I've been to one, but can't remember which one, though it was very good. I managed to find this about it:
"Port Lympne is the larger of the two and situated on a hillside. This means that there is a substantial amount of walking and some very steep inclines to go up and down. Personally it doesn't bother me but I know some people have got stroppy in the past, thinking it was too hard to walk round.
There's a mansion at Port Lympne that you can explore parts of, along with gardens (and an exceptionally long set of stairs!).
Port Lympne also has a safari tour, taken on large army trucks. You get driven around the park, through rhino and past elephant paddocks and into a large game enclosure. This is the only way to see some of the animals, including the giraffe herd. It costs an extra £5 for adults (can't remember the children's price). You get off of the tour at the top of the park so you can explore their insect/reptile house.
Howletts is smaller and flatter therefore easier to get around. The mansion there is not available to the public. It's unique feature is a walk through lemur enclosure where you have the little chaps run around you.
Animal wise, they are fairly similar. Both have tigers, gorillas (although the enclosure at Port Lympne is more impressive), rhino, virtually the same breeds of monkey, elephants, deer, gazelle, wolves and small cats.
Port Lympne houses some animals Howletts doesn't - namely barbary lions, ostrich, giraffe, zebra, meerkats, leopards, red pandas, baboons and a cheetah.
Howletts at the moment have fewer unique animals, but they do have 3 10 month old tiger cubs right now (the ones at Port Lympne are older) as well as a very young elephant."