I don't know the difference in the bars, but I do know that SlimFast isn't a ketogenic diet like LL, I've done SlimFast before and I was hungry all the time, it was horrible, so personally I wouldn't risk it as a) it could affect your losses and possibly hunger, and b) from memory the SF bars are much more food-like, and that could lead to wanting real food more, at least the LL bars taste suitably fake enough to not make you want more!
As for price, LL costs what it costs, dunno about other people's LLCs but I know ours won't let you simply buy 7 less packs each week and cut the cost, he was really funny with me one week when I said I had some left over and could I just buy a few days' worth for that one week, I was struggling with money that month and so in the end he did let me but he said he'd only do it that once, so even if you wanted to have the SF bars you'd be spending more cash as you'd have to buy them as well.
Also, from my point of view, if you're doing LL, then do LL, it's playing around with it that leads to problems, I should know it's the reason why I started to mess up so much and spend a year going up and down week to week, because I though I could just "tweak" LL to my own design. It doesn't work, it just doesn't and it doesn't help your head either to not be '100%' on the plan.
Of course this is just all my opinion, so sorry if that sounds like I'm being a downer Ralph
