If you plan to use a callback service in conjunction with a local prepaid SIM or if most of your calls will be incoming, then you should select the local carrier based on who has the best coverage, particularly if all the carriers' SIM packages run about the same price. Since the premium paid for calling a foreign cell phone from the US is almost always uniform for an entire country, you will gain nothing by picking a carrier with better calling plans but less coverage.
On the other hand, if you plan to make many outgoing calls from your prepaid SIM, you should carefully consider the different carriers' calling options and even different options offered by individual carriers to decide which best fits your needs.
When should I roam rather than buying a prepaid SIM?
"It is not a given that it is always cheaper to use a prepaid account, but it does not take many calls at $1.24+/min to make it cheaper. The formula, which I have developed (and humbly dubbed "Newcomb's Cypher"
is:
Break_even_minutes = Bare_SIM_price/(Roaming_rate - Prepaid_rate)
Subtract the cost of a typical prepaid call from the cost of the same call while roaming. Divide the result into the cost of the prepaid SIM, discounting it by the value of any included minutes. For some combinations of roaming and prepaid you will find that it only takes a few minutes of use to pay for the prepaid."