I would honestly try a a bottle. My DS was ff nearly from birth and we started introducing a free flow cup at 6 months, but there is absolutely no way he would have ever taken a full formula feed from a cup...he could barely manage the thing and didn't really get a hang of it until closer to 8/9 months. He also would generally just not take formula from a cup, only water.
It's a pretty big transition so I really think that breast to bottle is your best option. A bottle is more like the breast than a cup, still comforting etc. I think if you introduce a bottle you can still offer a cup, but more for exploration rather than actual feeds. We kept the bottle until 1yr and he managed to learn how to use a sippy during that time perfectly fine. I would not expect a 6 month old to be taking full feeds from a cup tbh. I'd try a bottle and see what happens
As for straw cups, we used the Playtex ones. I did find if you had the lid on too tight you could hardly get anything out because it would push the straw to the bottom of the cup and stop the liquid from getting into the straw.