To be fair, it's impossible to say that Firefly had hit its peak when it got the axe, based upon Whedon's past history. His shows are slow burners, and the first season is never the best(with the obvious exception of Firefly). Would anyone have been satisfied with Dollhouse(if you liked Dollhouse) if it hadn't gotten a second season? What about Buffy? Heck, I would go as far to say that Whedon might not have even had a career if Buffy had only its first season to stand on(which was formulaic monster-of-the-week business).
The Sarah Conner Chronicles were certainly great, and I wish it hadn't gotten canceled, but I don't think it had more potential(though not necessarily less either) than Firefly. I will say that I found the direction they were heading questionable though(traveling to the future the first time was a way of modernizing the show without breaking continuity[such as it is in a show with time travel{yes I'm nesting parentheses and brackets in the English language}], the second time was just a gimmick, IMO). They might have turned it into something great, but its equally likely that they were coming up with story hooks via a repurposed D&D loot chart and dice with that one.