The only answer I can provide is that when you get a new phone it will come with a new SIM. The number will be transferred to your new SIM.
base on this document Pay in Advance (Prepaid) plans the difference between "Simple Choice plans vs Simply Prepaid plans" is that with Simple choice you get less data bucket with international services, while Simply Prepaid you get more data bucket but no international services.
Simple Choice : $50 2gb, with calls and text to or from Canada or Mexico and Unlimited International text messaging
Simply Prepaid : $45 4gb, no international service, so you can only make calls or text in US.
So it really depends on what you need, simple choice is still a good deal because you don't have to pay extra for international services, although with simply prepaid with more data you can call or text using app's like fb messenger through internet.