It would help to know what your intended use is.
If the intended recipient is very small, especially if they are a girl with tiny hands, then the Mikro is definitely the way to go. It's the smallest scale (28", the rest are 30"), and probably the best built of the batch.
I always thought the Bronco was a fairly well built instruement, though the tuners are functional, but cheezy. Stock, it's definitely a thumpy one trick pony.
The Gretch has a good look, but I've played several, and it always seems like your paying more, but not getting more.
The SX is the cheapest of the batch, and based on my experiences with SX full scale instruments, it's likely to be built as well as the Gretch or Bronco. It might be nearly as good as the Mikro, but the fretwork probably isn't nearly as nice. It also wins the "look" in my book.
Sum up, if you're buying for a young child or very small female hands, I'd go with the Mikro. If you're just looking for a short scale to rock on, I'd go with the SX. They're both good instruments, and more tonally versatile than the others.
Other budget Short scales under $300 to consider:
Epiphone SG basses:
The Eb0 is very cheap (rediculously cheap used), and gets a fairly good mudbucker sound. Looks simply awesome! The
Eb3 gets you another pickup for 100 bucks more
The Douglas, Rogue, and Jay Turser Beatle basses. They are all very affordable quite lightweight and comfortable to play, and even if you move to better instruments later, it's always good to have a beatle bass in the stable. The
Rogue surprisingly seems to get very good marks, not for amazing quality, but for really nailing the hofner vibe.