GS would work with a single 30T chainring, but I'd still go for the long cage SGS. The only downside to a long cage might be snagging debris -- twigs, long tough grass, etc. That might be an issue for some hardcore off roaders in some terrain. But I'd still prefer the long cage. It's more versatile.

The only time I've had anything snag in my rear derailleur was when wind from a sudden storm front blew a plastic bag into my RD. It's pretty rare. Wouldn't deter me from using a long cage RD.