Hi Tanya,
You can do both. It depends on the exact scenario. The two methods you mention are both very sensible. Flowfield is maybe easier while GET takes more code, but gives you more freedom. If it is a List form/page, then be careful adding Flowfields to it, as it may indeed affect performance. On the other hand, so would GET. And with Flowfields, depending on the version, some work has been done to optimize calculating flowfields on pages.
So in short I think there is no black and white here in my opinion but if it is on a list page then just make sure that the flowfield / GET is as optimized as possible.
