I decide by talking to other families, reading reviews and prayer. Then I will usually try to purchase our choices 2nd hand. That way if it doesn't work, I haven't invested a huge amount of money. Also, I can always pass it along to someone else if our needs and expectations aren't met. It's just a decision. I can always make a new decision.
I have no suggestion for you on specifics as my children are a bit younger than yours. But I do hope you find a good fit for both kiddos. :)