Thank you for coming here first as there is much to learn and consider regarding either Parrot! I will cover the important points regarding Amazons and will let another member cover white capped Ponius and/or add to Amazons.
But first, welcome to Parrot Forum!!
Please read with 'understanding' the two Sticky Threads at the top of the Amazon Forum and especially the Thread about Understanding Amazon Body Language as Amazon's 'expect' you to understand their body language.
It is important to understand that the most popular Amazons (The Big Hot Three) are at or near the top of the Parrot species that are re-homed by new-comers to the Companion Parrot World. Most are drawn by their appearance and possibly, their abilities to talk. Rarely is this a good place to start when considering an Amazon.
In the Northern Hemisphere, we are coming off one of the most powerful Hormonal Season ever seen by even the most experienced Parrot Owners. It has been even worst with those owners of Amazons and especially the Big Hot Three. During this Season, one's beloved Amazon can become a very aggressive Parrot
The most common error that an individual makes is to walk-in with their 'Want' list! Dump the list as with the mid to larger Parrots, one really wants the Parrot to choose you!! The reason is very simple, the first goal is for the Parrot to develop a trust relationship with you! Parrots understand this deeply as they choose based on whether they have an emotional connection with you! It is possible to walk in and simply buy this or that mid to larger Parrot. But it is a very different story when that Parrot has no want to be yours!
General Point: You do understand that there is a size difference between the two Parrots you are looking at? Yes?
Prior to proceeding, you need to provide a Parrot Rescue your time 'free' to learn about Parrots first! Yes, start at the bottom clearing the facility, Parrot cages and stands, then work up to clean and replace food and water dishes. And, while all that happens, you will find Parrots that truly dislike you and others that are kind of okay with you. During this process, you will likely learn that Parrots can and do bite and can bite really hard!!
There is a vast amount to learn.
Again, welcome to Parrot Forum!!