You can no more overcharge a Li Po than a Li ion. That is to say, both will have charger circuits that protect against this.
There are many articles on the web about this, but to be honest to the end user there's not much difference between a lithium ion battery with a liquid electrolyte and one with a polymer electrolyte (yes, Li-po batteries are a type of Li-ion battery). Polymer type can be made thinner, but cost more. There are differences in stored energy density, but you don't care because the battery is part of the phone (i.e. it doesn't matter to you what fraction of the phone's volume is taken up by the battery). There's no difference on how you use the device as a result of this choice, and you may well have use Li-po before without knowing it. Basically this isn't something that I'd base my choice of phone on.