I don't know much at all about BigReactors, but its the more popular of the two from what I can tell, and it seems to be much easier to get into.
The first drawback for a beginner and ReactorCraft is that it needs you to learn RotaryCraft in some detail before you even touch it.
Past that, you need to add a decent amount of RotaryCraft and ReactorCraft infrastructure to get started. The mod strives for realism too, so there are several steps before you can start creating power.
And then once you are creating power, its a bit tricky to transform that power into RF; specifically, the converter block (rotational Dynamo) only outputs around 12k RF/tick, but you're generating on an order of 160k RF/tick for a small reactor, so you'll need to split that power up 14ish times before converting...
If you don't chunkload your reactor and its water source, its easy to incur a meltdown. Dealing with the waste can be a pain.
And after all of that, if you're not careful with the steam flying around (condensing all of it), it can really hurt your CPU performance.
Basically I gather BigReactors was just made with RF and regular end-users more strongly in mind