I'm guessing that's fast fermenter. I have the 7.9 gallon one. It was my very 1st fv I bought. I've never had the Speidel so I can't say . I can only tell you pros and cons of the FF.
Pros - good size , perfect for 5 to 6.5 gallon batches. Has the collection ball so you can dump yeast if you'd like . The lid opening is big and makes it easy to add fruit , wood cubes , hops ect.... and makes it easier to clean. It's solid and has the jacket you can use that works pretty good.
Cons - Bulky and oddly shaped to carry. It won't stand on its own so you have to lug the rack with it from point A to point B. The lid can be a booger to close right , it doesn't come with a spigot . I added one later which is a must. The transfer hose at the bottom is big , too big imo. With the valve and the collection ball at the bottom you have parts that have potential to leak . I never did but others have had issues. I've learned that I hate brewing anything with fruit in it. The conical bottom make the fruit drop and plug up the spigot area. When making sours with Blueberries I use regular carboy because the fruit spreads out nicely and doesn't compact itself too much.
I know there's probably more that I'm forgetting. I don't use it much but when I do it works . If your wanting to stay plastic and you only brew twice a year I'd probably stick to buckets . You can buy a bucket for 5$ add a spigot for 3$ drill your hole for an airlock and your done.