PBP is hillier (overall) than LEL and the time limits are tighter, you get 40 hours to do the first 600km to Brest and then 50 hours to get back. There's also the language barrier.
On the other hand, you've got ten times as many people riding PBP than LEL and the towns/villages along the way make a huge effort to support the ride, it's a big thing for them. All of the towns/villages on the LEL route had absolutely no idea what was going on.
They're different rides so it's hard to say one is harder than the other, but just because it's 200km shorter doesn't make it any easier.
Like yello I'm undecided. I did a BRM 400km ride last weekend just in case I want to but baby GB makes it difficult to get the time to put the miles in and then, next year, do the 200, 300, 400 and 600 qualifiers and then the ride itself. We'll see.