mcdonacw

I am looking into purchasing a GPS and noticing that they can come with the internal memory or with the option of a memory card slot. I am currently looking at the Dakota series by Garmin, as interested in mainly using it mainly for hiking, but like that I could use it in my car and on my bike. So I would want to have topo maps and street maps on it. Is this enough for the internal memory or is this why they have memory card slots? My wife and I travel the around and know that we would need the different areas for topo maps. With the devices that just have internal memory, are you able to remove certain maps and add others to maximize the space you have?