Make sure that you have provided for your children financially. Don’t choose your guardians based on financial circumstances and don’t expect them to bear the financial costs alone. Good parents make sure to provide for their children - in life and in death. Consider purchasing life insurance. Create and fund a Family Living Trust. Make sure your guardians have the resources they need to raise your children as you would.
As for deciding who you should choose as guardians, only you can figure that out but here are a few things to think about:
Will this person love my children the way I love my children? Believe it or not, not everybody loves your kids. Hard to imagine, we know, but...
Will they give my children the same type and amount of affection? Think about whether you're the "huggy" type. Are they?
Will they respect the wishes and desires I have for my children? First, they aren't mind readers so be sure to document that stuff. Second, think about things like family traditions, holidays, relationships with other family members. If your sister hasn't spoken to your parents in 10 years but you want your kids to know them, will she respect that and put aside her own issues? Families are fun.
Do they hold the same parenting philosophy as I do? Vehemently opposed to spanking? Make sure you aren't leaving your kids with someone whose first order of business will be to get a new paddle with your children's names inscribed.
These are just a few things to consider. There are soooo many but one thing is certain, money should not be your priority when choosing the people who will raise your kids. Love should be first on that list.