Ah flixel is realy better but I have not og it to download to my computer till not to long ago so I have not used it a lot, but flashpunk is based of of flixel
So it will be easy to look though the docs a figure it all out...
But a good library is makes a differnce from weaks of time to a weak and gives you time to add more feutures to your games, plus a forum to help yah out a bit, but I recomend learning how it all works and not limiting your self to only being able to program with the code library, but hope the libaray helps!
But how are you in spriting? I sugest going to the flash dojo (theirs a link to it on flixel in the help section, it is made by flixel and flashpunk it has great help there) and looking at the graphic help there, he explaned the basic tricks of the trade realy well! Like the red,blue tricks and placement tricks mostly, but helpfull. Also what do you use as your graphics editor? I use paint (simple but good) and used to have paintshop pro but lost it... Big oops(good thing some one gave it to me for free)... but now I use GIMP 2, it is a great program with nice layer,channaling, and such suport. Plus it free that alwasy good! I would say it is almost just as good as paintshop pro but with an easyer user interface, it does not lack a thing for spriting I just made some thing with it to day and it was great!
Well see ya round and good luck!