I personally love my Addis-I do also have the interchangeable nickle plated Options from Knit Picks, but my favorite is still the Addis. I find the tips on the Options too sharp and pointy, and much prefer the gentler, slightly more rounded shape of the Addis. If money were no object, I’d definitely have all Addis The Options are a great value, considering all the combinations possibilities you get though!
If it were me I’d start with a size 6, 24" Addi turbo. But really its hard to say-I knit rather loosely and often have to go down at least 2 sizes to get gauge. So it honestly depends on how you knit and what project you want to start with. For baby booties, gauge will most likely matter, unless they’re for charity. For a scarf of course, it won’t make a difference really in whether it can be worn or not.
If you’re going to be doing a baby blanket (unless you have more patience than me, lol) I’d go with a bigger needle! Maybe an 8-10 or even bigger, on at least a 24" (or 32" would be better for a blanket!) cable. Using your yarn held doubled throughout would also help; that way you would finish more quickly than with a smaller size needle.
I’d just pick whichever size would best suit your first planned project with them, to get a feel for whether you’re going to like them or not. I hope that helps some!