The difference between 餐馆 and 饭店 is:

餐馆 is only meant for a place where you consume certain cuisine, i.e. a restaurant

饭店 can mean either a restaurant or a hotel, depending on whether the place offer accommodation as part of its business or not.

if you say, "I'm going to xxx 餐馆", people understand that you are going there for a meal only, if no other specific purpose is mentioned.

if you say, "I'm going to xxx 饭店", people may think you're going to eat there (if there's a restaurant at that place) or you're going to stay there for a night (if it's a hotel).

Last but not least, it's more common nowadays in China to say "酒店" instead of "饭店" to name a decent hotel.