Well, so far, Martin has clearly shown more. He wins a lot of races. He's an excellent climber, has a lot of punch over hills, and has a very good sprint from a select group. If you effectively miss half the season and still finish in the WT top ten, you are pretty good. Also, he's insanely attacking, which may not be the best way to maximise your palmares but is certainly the best way to be popular with the fans.
He has two very big problems. His time trial is poor and, as mentioned above, he has to date been rendered uncompetitive in what should be the part of the season most suited to him (Ardennes, on the one day side of things, or the Giro out of the GTs). We don't know if he will be able to address either of those issues. If he becomes competent in the TT, then he's a legitimate threat in GTs. If he doesn't, then he isn't. As for the allergies, it must be enormously frustrating to come second in Lombardia but know that your nasal passages will likely be swollen shut at LBL.
However, I think that the timing of the question is a bit unfair to Roche, as he's coming off a very disappointing and crash-disrupted season, where he was both racing all year long and never able to peak properly. He did pick up a decent win at the end of the season. He is a little less ropey in the TT and he has better GT GC finishes to date. I actually think that he has a lot of talent, but tend to concur with the people above who have been saying that riding for minor places on GC isn't the best use of that talent. What's more, it tends to stifle his attacking instincts. Unfortunately, the economics of present day cycling seem to push talented riders towards going all out for that coveted 8th place in a GT.