Get the bigger one as a smaller one is useless for doing a chook, lamb shanks or a leg of lamb/roast beef/silverside. Slow cooking is so much more than just casseroles, soups and stews these days and you will get more benefit in the long run from a larger one. There is also the benefit of having leftovers you can freeze if you just don't feel like cooking one night or need something quick and easy to get out of the freezer. That said I would also like a 3.5lt one myself so I can do desserts.