If you only tasted one Pickering, how do you know it wasn't representative ?
Yes, Angie is a larger tree than Pickering. While Pickering has good attributes in its tree size, I would have to go with Cogshall for being a better quality mango. Personally, I think Pickering gets a little bit of over recognition due to its dwarf growth habit. While a well ripened Pickering is a good to very good fruit, it is not what I would consider excellent. A well grown, well ripened Cogshall can be excellent.
Will your sister be able to keep it pruned or is it basically plant it and let it grow ? What size tree are you looking to buy as this would make a difference. Also, where would you buy it, again, this would also make a difference.