I would go with the convertible, since it will last longer and you will need a convertible at some point anyway, so might as well put the money you put towards a base, towards the convertible. That would be my reasoning, anyway, and at 20 lbs, I would imagine your DD Is really heavy and awkard to carry around in a bucket seat, so there isnt' much point in using it.
I had a Graco SafeSeat with my DS and he was on the small side...I think only 17 or 18 lbs at a year. I believe we moved him to a convertible around 16 months...not so much because he outgrew it, but because it became uncomfortable and he seemed to like being more upright (rear-facing) in the convertible.
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