Honestly, as a Roy supporter myself, what we mostly fall back on is Roy's massive popularity in Japan (second only to Mewtwo himself), where Sakurai's going to be looking at mostly for character ideas. That and his ease of inclusion, possession of his own unique gimmick, his veteran status, and his continued appearance in the series thanks to FE13 DLC and SMT vs. Fire Emblem. While none of these are particularly strong points in Roy's favor, Chrom fails to match up to a lot of them, especially the popularity and uniqueness fronts (both visually and moveset potential-wise).
What Chrom has over Roy however, is not to be ignored. As the most recent FE lord and as the lord from the best-selling game in the series thus far, he definitely does have some weight behind him, even if he's more expected than wanted. However, he's definitely not a guarantee either. At the same time, neither is Roy. However, both of them definitely have a decent shot at it if there are only three FE reps. Four FE reps is definitely possible, given how big the series is in Japan, and if we did get four reps, I'm positive it would be Marth, Ike, Roy AND Chrom, but it's not altogether that likely.
In the most probable way I see it, Fire Emblem reps could go one of two ways.
If Sakurai goes to the Japanese Smash fanbase to decide FE, we'll get Roy as our rep.
If Sakurai asks Intelligent Systems who to include, or specifically wants an Awakening rep due to sales, we'll get Chrom.
Seeing as how there's already an Arena Fenox stage, I personally believe the latter is going to happen. However, if and when we get DLC characters (which is pretty likely given Sakurai's comments on the matter), Roy is absolutely going to be one of the first DLC characters brought in, as he'd sell like gangbusters and would please a huge section of the fanbase, while at the same time being very simple to implement.
So overall, most of us Roy fans agree that Chrom is more likely than he is, but we argue mostly because Chrom/general FE supporters continually underestimate his actually pretty decent shot at getting in.