I think the question comes down to how often and where you plan to use the keyboard.
If you already have the smart folio then switching back and forth between that and the Smart Keyboard folio is going to be a pain. If you only want a keyboard for occasional use then get the magic keyboard (which, in fact, is what I did and what I am using to type this). It also has the benefit of being a ‘real’ keyboard with better feeling keys and media control keys. Even the volume and brightness keys on the magic keyboard work with the iPad.
If you need a keyboard all the time, then maybe the Smart Keyboard folio makes sense instead of the smart folio, but for me I don’t use the keyboard often enough to justify the extra bulk and weight of having the Smart Keyboard folio attached the iPad all the time, and as I said above switching folios just to use the keyboard is awkward and annoying. With the magic keyboard you can just grab the keyboard, turn it on and start to type without having to switch folios.
One downside to the magic keyboard is that it really doesn’t work to hold the iPad and the keyboard in your lap as you type. The Smart Keyboard folio makes lap typing possible, so if you anticipate needing to use a keyboard while your iPad is in your lap frequently then you might want to consider the Smart Keyboard folio.