I would go for the A.fulica white Jade. fulica (inclusing all of their colour varients) are usually easier to keep than A.iredalei. There are folk who keep iredalei without problems but there are also plenty of people who struggle to keep them. Fulica are among the best starter snails aroud in my opinion. Easy to keep and very active, not shy of being handled (if handling them is your thing) and if you want to add another species to the tank later fulica make good tank mates to most other tropical species. They'll need to be kept warm day and night, but have a wider temperature tolerance to many other snails. As for size, many (but not all) white Jades are known to be smaller than other fulica.
I would also bear a thought to achatina reticulata, they are very active, have lovely shells, are tolerant of quite a wide range of temps and humidities (thought they like it a bit hotter than fulicas) and are something more unusual which are still easy to keep, although, you should bear in mind that all snails have different personalities so you can never be sure!