Not sure if you are likely to get a completely unbiased reply on this forum Diane
most people here are confirmed "Arilliacs".
Seriously though - it all depends on what you are looking for from a holiday.
San Stafanos has a long, flat, gently sloping sandy beach. Really great if you have small children.
Arillas beach is not so big, and tends to suffer from the sea shifting sand around, so sometimes it is really good and sometimes it is quite narrow in some places. But it is a really nice beach and you can walk to the far North and South ends of the beach which are both very secluded. These ends are favourite haunts for naturists, but it is not essential that you join in if you don't want too - just be aware that they are there if any of your party might be offended by such things.
Both resorts have great places to eat, and there is plenty of choice at both resorts to suit most everybody, although overall there are more places in San Stefanos as it is a slightly larger resort.
Neither resort has what you would call a "noisy" night-life, although both have a couple of good bars where you can listen to music and they sometimes have live performers.
It is possible to walk along the roads between the resorts, or there is a shorter route over the headland between the two bays, and there is a really great taverna at the top where you can rest and get some liquid refreshment, but you may find both of these routes a little too much after a big evening meal!
I am sure that many others will post their thoughts on this subject, this is a very active forum with many wonderful contributors, so you can guarantee that any information you are given here will be honest and sincere.
Meanwhile, to get a better idea of Arillas there is not only the camera on this site which will give you a good view of the bay, but there is also a website which shows a "virtual" tour of Arillas here:-
http://arillas.corfu360.net/You can also get a good view of San Stefanos beach from the San Stefanos Travel camera which is on their site here:-
http://www.san-stefano.grAnd a virtual tour here:-
http://www.agiosstefanos.corfu360.net/Whichever one you choose, you will have a great holiday.