The hat will probably be easier (not having seen the pattern) and will probably require learning a few skills that will be helpful when you take on longies.
My first project was this and it was great because it is a forgiving pattern plus you do most of the stitches you'll need on the other items. Now I'm doing the Tiny Bird's soaker (flat soaker like Poonico mentioned) and then I'll try something on circulars next.
Re: Knitters - which would be easier
Green Living Blog
for eco-information and fun giveaways!
The hat, but the longies wouldn't be much harder.