I think reason two is most logical, I mean after all, Ernie coincidentally gave away his Rubber Ducky for Bert... reason three DOES make sense slightly, but I don't know about reason one, I mean towards the end of that scene Bert almost cried.Well, Bert did chose to give Mr. Hooper his paperclip collection rather than his bottlecaps. This could be for one of three reasons:
1. He didn't mind giving away his paperclip collection, as long as he still had his bottlecaps.
2. He chose the paperclip collection to give away because it was so important to him. After all, this was for Ernie.
3. He figured (in his own Bert-ly mind) that the paperclips were worth more than the bottlecaps and he was doing Mr. Hooper a favor!