I work at Noahs Ark and think it would make a great day out but it depends on what you like the most Bristol zoo is also very good.
Noahs Ark Has;
Rhino, giraffe (and new baby), zebra, deer, and a lot more of the bigger stuff than Bristol zoo. Also a lot of play indoor and out.
Bristol Zoo Has;
Garrillas, sea lions & penguins (in a brilliant high tec. enclosure), nice gardens, and a big café, and one of my favourites a big walk through aviary.
They both have a lot more to offer than the short lists above but they are the most different items.
I have been going to Bristol zoo all my life and I would recommend it to everyone, it is small (12 acres) but walking round the well-established and well laid out ground it feels bigger.
Noah’s ark Is run by strong Christians and try to give the place a family feel it is also a working farm (200 acres is farmed and 100 acres is the zoo) it is very hands on and you can get the feel of the farm at the same time as seeing tigers and letting the kids go on some great adventure play.
You can’t fit both in a day

If you have children you would be hard pressed to get them round all the play areas at Noah’s ark fit in an animal show or reptile experience, or tractor ride, with enough time left to see half the animals in a day.
You will have to do both

make two trips or stay overnight. Noah’s ark isn’t open on Sundays.
This may sound like an extravagant week end but how about;
Saturday at Noah’s Ark Zoo farm an overnight stay at Aston court (see our web site for a special rate or book something on line) take in a show or go to the cinema Saturday night and do
Bristol zoo on Sunday.