Depends on what you want from the snake. Both are nocturnal, so you won't see them much. But, if you want the opportunity to do a bit of decorating in the cage, you can do it on a smaller scale and have it be sturdy enough to withstand a rosy's nudging.
If you live in a drier area, a rosy would be a better choice because they have limited humidity needs - keeping them on a thick (and I mean THICK) layer of sanichips or shredded aspen should be all that is really needed to keep them shedding well.
Ball pythons will get larger - 3 ft or so for a small male up to 5 feet for a big female!
An average adult male rosy is going to be around a foot or two long, while a female can get up to 3 or more feet (we've found a 3.5 foot one at my house before). The male can definately stay in a 10 or 20 gallon enclosure, but a larger female (or larger male) will need something a bit bigger.
You can keep an adult ball python in a 20L enclosure and it will be okay, but an adult tends to do best in something with the floorspace of a 40 gal breeder tank.
How is the heat in your area? During summer, is your house in the mid eighties? That's perfect for a rosy boa, while a ball python would still need a little supplemental heat.
Both rosies and ball pythons are likely to go off feed in winter due to the season, although not all rosies do it as regularly as ball pythons do.
Rosies don't need as much cage furniture as a ball python would - giving them enough substrate to burrow in usually makes them happier than any number of hides would. Ball pythons are going to need two hiding places - one on the hot side, one on the cool, so that they can pick which side they want to be on.
Rosy boas also have the choice of locality - you can pick a nice baja rosy that's chocolate brown and cream, or you can pick up a san gabriel, which would be a metallic orange and blue, or a coastal rosy, which would be slate blue with grey. There are a lot of varieties in rosies and they are all in the same price range. Ball pythons do have morphs but they often cost hundreds of dollars more than the average baby.
Hope that helps!
~jenny
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"We call them dumb animals, and so they are, for they cannot tell us how they feel, but they do not suffer less because they have no words."
- Anna Sewell (1820-1878)