Ask yourself how long you see yourself using this computer. If you are the sort that is always after the newest device, save your money now. It is probably that you will trade this computer in before we reach the point where 8 GB of RAM will be essential for a smooth experience. If you intend to use this computer for 4-5 years, than the extra RAM investment now will be well worth it to keep your new machine running well towards the end of its life.
Once you've chosen the model and screen size you want, you should always consider RAM and processor boosts to an Apple laptop if you plan to use the device for several years.