I have a FEL on my Branson 3015H. Which I’m not an expert at using, so would a box blade be easier for spreading dirt, gravel, etc….?
Both the box blade and the FEL take some practice to become efficient when spreading dirt. I find that using both of them together is most efficient. If you are spreading dirt from a pile dumped onto your driveway, a box blade (when proficient with it) will very quickly spread/grade the dirt, and in a very controlled manner. A hydraulic top and side link make adjustments 'easier', however those adjustments can also be made with your manual top/side link.
A FEL is okay for spreading dirt. It's a lot easier to become proficient with it so typically people are very comfortable using them (and it seems like they are always on.) You can back drag the FEL in float, and by varying the pitch of the bucket move more or less dirt with some simple adjustments.
For moving dirt over a paved driveway or lawn for example, a box blade is not very useful, you will need the FEL. However, if moving dirt over an area that is already dirt (such as unfinished lawn or gravel driveway your touching up anyways) then a box blade will carry a lot more material. I can carry a little over a yard of dirt in my box blade, but only 1/3 of a yard (at BEST) with my bucket. After moving many hundreds of yards of material with my tractor I found that scooping up a bucket full, then turning around and filling the box blade, was the most efficient means of moving the material moderate distances.
The box blade is my most used attachment, however it takes some getting used to. Once you figure out precisely how to keep it perfectly adjusted, you can do a LOT with it in a short amount of time.