Don't know how easy or difficult it is to switch modes between 8 and 18. However, the day may come when you look at a pic and wish you could really crop it.
When my dog was a pup, he went into a lake and when he came out, he stood with his hind legs in the lake and his front legs on a rock and looked quite regal. Unfortunately, we only had time to grab a pic from where we were standing before he came out of the lake.
Looking at the pic, he was a relatively tiny portion and the lake was everything else. It was a picture of a lake with a dog in it. If we could have blown it up and cropped it, it would have been great. It would have been a picture of a dog in a lake. But it was a 5 megapixel camera and when the pic was blown up and cropped, it was worthless.
My suggestion is to shoot at the highest quality the camera has available. If you need to reduce the size of the file later, fine. You can always do that. If, however, you throw away the detail in the first place, you can't get it back.