Is it better for a current Xfinity Mobile customer to buy a new iphone from Apple or from Xfinity?

No catch, several friends and I have already upgraded/replaced our iPhones and received our debit cards. Just order iPhone, pay in full, replace device, and so on. It's a little confusing and tedious. 

$250 Back: Requires upgrade of an existing Xfinity Mobile line to an iPhone and activation within 30 days of order. Existing phone must be paid in full if eligible phone is purchased online or pre-ordered. At least 50% must be paid if purchased at an Xfinity store, and an upgrade fee may apply. Must maintain Xfinity Mobile line with an account in good standing for 90 days following phone purchase. Visa Prepaid Cards are issued by MetaBank®, National Association, Member FDIC, pursuant to a license from Visa® U.S.A. Inc. This card can be used anywhere Visa debit cards are accepted. Prepaid card mailed to Xfinity account holder within 16-18 weeks of phone purchase and expires in 180 days. Maximum $2500 in prepaid card amount per customer/account. One card issued for total qualifying amount.

1.) XM does not support eSIM yet, if ever. It will provide a new SIM with the new iPhone. 

2.) XM will unlock you new phone and it's eSIM at your request after a certain amount of days. Maybe 30 or 90 days? 
3.) You are be able to activate a data or voice eSIM but it cannot be a CDMA carrier such as Verizon as iPhones will only allow one CDMA line. Since Verizon is in the process of phasing out CDMA, it is likely that you could add a non-CDMA Verizon line sometime in the future, or maybe even now, but it would have to be done in a Verizon store.