I own a 1990 Firebird reissue, so I can speak generally about them. There's pros/cons to both. Real vintage Firebird pickups have a twang to them, almost Strat-like quality but humbucking. All the recent decade Firebird pickups are way overwound, so they just sound like mini-humbuckers. Like a thinner, weaker Les Paul (which is not a bad thing, just works better as a rhythm instrument.) And actually, since I have an SG with P-90s, I can say the bridge pup of the Firebird and a bridge P-90 in a mahogany SG (similar to Junior) sound very much alike. The number one bad thing about Firebirds is the neck is so thin, if you strike a chord and lean forward you hear the chord go in and out of tune.
The Firebird has Tune-O-Matic, so you can get the intonation right. The Junior is fixed wrap-around - though I've been impressed that most recent wrap-tail Gibsons work just fine - even my Melody Makers play in tune. The Junior is one pickup, so you will just be rocking and not getting much in the way of any mellow, ballad or jazz versatility out of it. I'm using my experience with my SG Classic (P-90 / Mahogany) to estimate what the junior might be like.
I might side with the Junior. Affordable, solid, probably more useful all around. Firebird is nice to have, but expensive and will never make it as a daily player.
Good luck.
