Welcome from NJ! While I agree with most others that the patterns you have initially chosen can be challenging, I'm also of the belief that you should make something you like! Out of those 2, I would probably start with the triangle one despite the bias edges - starch is your friend! Unless you have a garment construction background the curves on the other quilt might be more of a challenge than you are currently ready.
I also agree with starting with a baby or lap-sized quilt to start so as to not be overwhelmed with sheer size of your project.
Personally, I would not recommend starting with pre-cuts as they can have their own set of challenges in addition to not gaining the learning/experience of cutting fabric in general. Then again, I'm just not a fan of pre-cuts but have used them when expedient. I prefer yardage and cutting to size as needed.
Yes, rulers will slip. Get some rubbery stick-on dots for your rulers. Put one in each corner. Works a charm. They can usually be picked up reasonably at your local hardware store or JoAnn's also carries them. Well worth the investment.
Best advice I can offer beyond the above - when purchasing rulers, purchase all the same brand. If you are going to go with the triangle quilt as your first project, you'll probably need a Creative Grids triangle ruler. If that is what is recommended - then purchase all of your other basic rulers in the same brand as they tend to measure differently among brands. You'll progress and flow as time passes but at least start out with a couple of basic of the same. Or at least use the same brand throughout a single project.
Remember this is supposed to be fun. Just enjoy the process and forget the race!