the best thing to do is to look under their skirt, by finding out more about the actual trading costs.
Trading cost is influenced by mainly spread and commissions, but you should also pay attention to swap mark-up and conversion loss. These cost types are less known, however
they can raise your trading cost substantially.
If you want to get a good comparison of broker costs, I'd suggest you to take a look at the TraderProofer (https://tradeproofer.com/
), they have a cost comparison engine which takes all
types of costs into consideration, you can even personalize your search accorcing to your trading style.
I hope this one helps for you.