Hi there,
I switched to the seat at about 4mths. If anything, the bassinette component would provide more arm space than the seat. The main difference is that baby can see out of the seat component better. I would keep your bub in the bassinette for a bit longer - it is much better for sleeping and probably more comfortable and roomy for bubs. I would wait until your baby has good head/neck control before moving to the seat (ie. at least 3mths).