If you can get a better price on the market instantly then definitely I would go for that. But I have checked this for a while now, since we were on the steem blockchain and always I got a better rate by converting than either the internal market or on exchanges.
On the internal market people are always selling at 3-4 cents higher. And on exchanges they take exact value of HBD. But the conversion feature takes hbd as $1 even if HBD is actually 60-70-80-90 cents on the market. The only situation un which exchanges might give a better rate is,if the HBD price is significantly above $1. This is just from what I've observed over the,past couple of months.
See the dips in my earlier graph. Manually this is hard but you can automise it with a bit of beem :o)