Personally speaking, out of the two, I think I'd find the 5lb one easier and I'd be more likely to stick to it. If you're already struggling with just 2 or 3 days, having a 90 day challenge will probably overwhelm you and you could end up self-sabotaging, without even realising it. If you muck up at any point before that 90 day end point, it'll be really dispiriting and you're much more likely to just have a really frustrated "well screw it!" kind of reaction.

I'm saying that more from my own personal perspective, rather than basing that on you Ms Arielle, because I'm still new here and I don't know you. But I think when trying to break out of bad habits and create good ones, slow & steady incremental change is the way to go. Extend your challenges bit by bit after you've hit a smaller one and build from there. Monday is D-Day for me, so I'll probably be feeling like death and dreaming about everything carb-tastic, for most of next week. Just gotta get my stubborn head on me and hopefully power through the worst of it.