Helloo and Welcome!!
I think they are both equal in the learning area, if you are a new flier. Both are very different in the way you fly them. Quads will be more expensive (higher cost of lines [x4 instead of x2], handles), and duals will be a touch cheaper (correct me someone if Im wrong here) with being able to buy different 'classes' (beginner, intermediate, competition classes).
I started off on a dual line kite and had a blast for a year on them before getting my first Rev last week.
But its really a matter of taste.
If you know someone with a dual or a quad, give both of them a whirl.
If you just 'want to get out there' and fly, go pick yourself up a dual line kite -- "Level One Jump", a "New Tech Cherry Bomb", a "Prism Nexus" or a "Prism Quantum". All of those are under $100 (the "Jump" is around $120, but well worth it in my opinion), and will get you out there flying in no time.
You can troll around the internet and pick up a Revolution SLE for around $200, and may be able to find lines included, so all you'd have to buy are handles. It's WELL WORTH the investment in my opinion -- Rev's are just so darn fun!!!
Hope that helps (although I may have made your choice a little harder. lol)