I think they always like to go with LMP dates as they are usually more accurate than a scan coz all babies grow at different rates.
I'd go with the 6th. In between.

I had 11, 12 & 13 April with DD1. I went with the 12th. She came the 20th.
I had 25 & 29 May with DD2. I went with 29th May, she came June 5.
DS was the 6 & 10 March, he came on the 8th.

I think better going the later one so they don't think you are over due if you aren't, but as I said LMP is usually more reliable.