Going to the Mediterranean or the Seychelles? If you are a competent sailor, both the ICC and the SLC are relatively simple to get and equally welcomed by governments and yacht charter companies.
Here are the differences
The ICC (the International Certificate of Competence) was created by the United Nations under Resolution 40. Only Countries who signed the resolution can appoint a sailing body in their country to issue the ICC. The United States or Canada did not sign the resolution, thus no sailing body in the USA or Canada has been appointed by USA of Canadian Government to issue the ICC. The RYA (Royal Yachting Association) in the UK is one such company. It was appointed by the UK government to oversee and issue ICC's. The RYA can issue the ICC to USA citizens and Canadian citizens through an RYA school. One trouble is however that there are only 3 locations in North America - Toronto, Halifax and Florida. So unless you are close by to those locations you're going to need to get on an airplane.
The SLC (Sailing License and Credentials) is administered by NauticEd. In the USA, NASBLA is the government administration authorized by each state to oversee boating licenses. NASBLA was further instructed by the US Coast Guard to oversee verification of sailing training bodies as to adhering to the US Coast Guard American National Standards for Sailing Instruction and Assessment. This link verifies that NauticEd has been recognized by NASBLA. Following this the Government in Croatia signed into law that the SLC is an acceptable Sailing License for Croatia. Croatia being the toughest, all other mediterranean countries and the Seychelles followed. Additionally, all yacht charter companies operating in the Mediterranean accept the SLC as a legal and valid license.
The Advantage to the SLC is that there are many more schools in the USA and Canada that can perform the one day on-the-water assessment of skills that is required as part of the license.
To gain an ICC you should still do the online Bareboat Charter Master Bundle of courses since the assessor will ask you lots of technical theory questions and will fail you if you don't know the answer. After you have brushed up on theory, you do a one day assessment on-the-water at an assessor at an RYA School.
See this infographic here: