I had an 8 week scan and I needed a full bladder. It wasn't internal, but they told me when I booked that the later I left it ie 8 weeks instead of 6 the less likely I'd need an internal. Turns out I needed an internal one at 12 weeks in addition to the normal one so I didn't miss out in the end . Good luck.
At 6 weeks week you might need an internal scan, but might not. My last 6 week scan they were ok with an external scan. I would go with full bladder as better chance then of not needing internal. If they can't see clearly they will ask you to empty bladder before doing internal scan. Good luck.
For the internal ones I've had so far (at 4 weeks, 6 weeks, 8 weeks), she said it was better to have an empty bladder, but yes, for the abdominal ones, a full bladder! My doctor told me that at that early stage, there's very little they can see with an abdominal scan, so they tend to use an internal one.
It might depend on the hospital. I had scans at 5 and 7 wks and both were done internally (and I was skinny at the time so wasn't to do with that). Best to go with full bladder as they wanted me to wee just before the scan so bladder completely empty - you can ask when you arrive. Also you may have to wait? Good luck, I hope it goes well.
In the Past I have had to have both internal and external at the same scan. Arrived with full bladder, had external scan, went for a wee, had internal scan. I would go prepared with a full bladder, its quicker to empty than it is to fill up.