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when you send BTC to your wallet in bitshares using the blocktrades bridge it automatically buys BTS with your BTC - what is the benefit of your method above cheers ?

You can set a low buy order to receive more Bitshares. That is if it executes before Bitshares rises in value vs. Bitcoin. Using the blocktrades must make a market buy order at the current sell price which might be much higher than placing a buy order right above highest current buy order.

ok that makese perfect sense ! so to send BTC but buy BTS later do you choose Gateway - OpenLedger instead of Blocktrades ?

That is correct.