Quick answer: Webb
Now the long part:
Webb: He is younger his times are just a tad slower than those needs to be a champion, he is close to 3:30. Webb seems very good at running races flat out. He will need to learn how to deal with the cut and thrust of competitive 1500 meters. However he has the initiative to break a race open with a drive (olympic trials). His olympics was a fluke, he has typically run much better than that. He has a fairly average kick but also lacks any true weaknesses and has a good drive finish, as El G has shown a drive can be super effective. With El G done or on his way out and everyone else fairly balanced the 1500 looks to be wide open for the future, leaving chances for solid guys like Webb to win. I think he has shown potential for even further improvement.
Mottram: Mottram has a tough order. He has run 12:55, but that is fairly average olympic finalists now. His 3:48 is good but that doesn't put him above all the 5k guys, most 5k guys have rarely made serious attempts at the distance. Mottram finished close to Bekele, but Bekele was comfortable and beating Bekele is lightyears removed from finishing close to him. He also has Kipchoge to deal with, Kipchoge is flat out better and has the kick to defeat Bekele. Songok is a new threat who has shown great finishing speed now that he is accustomed to the distance. Mottram is a very average 12:55 runner (if such a thing exists), good but not great in all respects. He needs a titanic kick to win, look at the guys who've beaten Bekele; Chebeii, Kipchoge and El G, all those guys are utter monster finishers at 5k. Against the level of competition Webb is facing, Mottram would have as good a chance as any, his competition is better.
One key point: In Webb's best races at 1500/mile he won over in Europe running 3:32 against Saidi-Sief, Heshko, Lagat and Prefontaine. Whereas Mottram's biggest races have at his prime distance have been in losing efforts ( to an old, over the hill Geb, and a cruising Bekele). Those losing efforts show where Mottram stands, the guys he has to beat are so damn good, even on his best day he can't touch them. Webb has beaten, at one point or another, almost everyone he would need to get a gold, just not in the championships.