Just had a conversation with my Surgery employed Pharmacist. she asked me if I would object to switching from Simvastatin to Atorvastatin as the Simvastatin was rather an old drug. I was put on it around 2007. I told her to go ahead ... no problems.
The following quote is what I found during an ultra quick search. Probably shedloads more around that I haven't time to explore.
"Atorvastatin works for much longer than Simvastatin; it has a longer 'half-life' Atorvastatin appears to be more effective than Simvastatin at lowering LDL cholesterol (bad cholesterol) Atorvastatin has a lower risk of side effects compared to Simvastatin; such as muscle pain and drug interactions".