Yep, at first, you just go for that little baggie, but that's not enough for very long. Then, you step up to a pound. Then, it's more than one pound. The next thing you know, you have a whole shelf of an upright deep freeze packed with 13 different hops. Then, sud*denly there's a ter*ri*ble roar all around you, and the sky becomes full of what look like huge bats, all swoop*ing and screech*ing and div*ing around your brewing rig. You're in, now. You're in for good.
As for the difference between leaf (or whole or whatever you want to call them) and pellets, I find it's more a matter of personal taste. Leaf hops are more romantic, more attractive, and they tend to smell a little better when you rub them. However, compared to pellets, they have lower utilization (roughly 10%), take up more space, are more difficult to measure accurately, and are not as stable for long storage. Some folks say that leaf hops are better than pellets for dry hopping. They may be right about that (I don't know -- I really haven't compared the two that strictly), but pellets work well.
Just about every time I buy hops in bulk, I buy pellets. The good points outweigh the bad for me. I used to brew exclusively with leaf hops, and I still do not mind using them one bit. I certainly won't go out of my way to use pellets when perfectly good leaf hops are ready and available. I can go either way, but all else being equal, I tend to go with pellets.
On storage, heat and oxygen are your hops' enemies. Keep them cold, and keep them away from air. The freezer is the best place for them. Baggies work, but vacuum sealing is much better. You cannot stop degradation, but you can slow it down a great deal. I keep my hops in the original package and then vaccuum sealed in a Foodsaver bag in the freezer. I have some three-year-old hops that are still quite fit to use.
TL