we've been using the baby bjorn potty since dd was 4 months old and we've also tried a couple potty seats on the big toilet. she's a big 20 month old now and i'm worried the little baby bjorn is uncomfortable for her. she's great about getting on and off it herself when she has to go (and naked) so i'm thinking about switching to one of those standard potty training toilets. any suggestions for what to get or if it's even necessary??