Hey Flander,
From my perspective those two options couldn't be more different, so it really depends on what you value most. I've always thought that decathletes are the best overall athletes. They do so many things at close to elite levels, and could probably be put into just about any physical/athletic situation and come out on top. Personally, these guys are closest to the physical ideal. But if you have a chance to be elite at something, than that factors in independently. I've never been in a situation where I had a chance to be the cream of the crop at something, so I can't speak to how important and motivating that is.
I understand why everyone is urging you towards powerlifting here, going for glory and all that, but even though you might not reach the same pinnacle of sport in decathlon, the training would probably set you up to be a much better athlete for a long time (if that is what is important to you.) The training would also be more varied, and possibly more fun, depending on what you like.
A little bit obvious I'm sure, but that's my two cents. Decathletes are badass.