Pre 2000's, I would have said it depends on whether you have children, and on the time of year.
Marwell isn't the best in winter as virtually nothing is allowed out on the grass (although this has improved ever so slightly in recent years).
However, Drusillas to me lost its way when the current owners took over. Before around 1998, you had some of the most innovative exhibits in the UK: Beaver Country, World of Owls, Life on Earth, Monkey mountain, Penguin bay, Meerkat mound. Those with knowledge of these exhibits may not think of them as particularly special, but thats because so many of the ideas pioneered at Drusillas have been emulated many times throughout UK collections. Many of those exhibits have been shrunk down to make space for other species, or restocked with fairly random species unconnected to the original theme of the exhibit IMO.
I also feel that Marwell has lost its way but that has been discussed elsewhere in great depth.