My daughter did FSCE and GL last September. The schools usually have one test, but one of the schools switched to FSCE a year in advance of the others in the area a few months before the exam.
She passed and was offered places for both, but a few observations.
Firstly, we kept focused on GL as it was at a higher level including year 6 content. We then did some mock FSCE papers (albeit not true mock papers, but published look-a-likes) which helped with the format preparation.
My DD found the FSCE much easier in terms of difficulty on the day. The challenge with FSCE was sheer volume of questions. The mock papers were punchy in terms of questions and time. The actual paper had about 25% more questions with a couple of minutes less time to do them in.
The other main difference was that GL was all multiple choice, whereas FSCE had multiple choice and write the answer sections. It didn't feel like it would help kids with dyslexia, as a couple of kids in her exam had some extra support and came out saying they struggled.
DD's feedback was that GL was a tougher paper that tested your knowledge. FSCE was easy, but a test of how quickly you could answer questions.
The idea that FSCE can't be prepared or tutored for is not realistic based on our experience. A child who hasn't been prepped on exam technique, quickly answering and moving on etc, is going to struggle. There was nothing in the FSCE exam wasn't already covered in the GL Exam prep, so her work for the other entrance exam definitely helped a lot.
Finally, there are different versions of FSCE format in different areas. Where we are, it was two papers covering maths, English and some verbal reasoning.