With a Travel SIM card, there are no nasty bill surprises when you return home. You know exactly what you get upfront and know exactly how much you are spending.
International roaming rates vary depending on your home network roaming agreement with each international network. It can be as much as 300% more expensive than using a travel SIM and, not only do you pay for outgoing calls, but you also pay for incoming calls and voicemail retrieval. Since cross-network billing is delayed, an international roaming bill can reflect as much as 2 or 3 months after your return.