At least glance at the used Minolta 3600 HSD and 5600 HSD.
These have got much cheaper of late for no really good reason and a good used one can be an excellent bargain. The HSD designation (or HS(D) or ?) indicates that they have high speed multiple flash sync - useful on occasion.
Note that the ONLY Minolta flashes that work with the Minolta / Konica Minolta / Sony A Mount DSLRs are the 2500/3600/5600. There are numerous other Minolta Program Flashes that will not work fully with the dSLRs. There is an enthusiast add in microcontroller mod for the 5400 which makes it more or less compatible with some but not all of the DSLRs - but you are reasonably unlikely to encounter a flash with that mod.
The 5600 was Minolta's top flash and initially continued to be made by Sony after the takeover and then replaced with their current top offering. The 2500/3600/5600 will work on all Minolta and Sony DSLRs starting with the Konica Minolta 7D and up to at least the A77. From what I read about the A99 it may not be hot shoe compatible, but I'd guess that it is liable to talk the same talk.
The 5600 head pivots vertically and horizontally. The 3600 head only vertically - a triumph of marketing over 50 cents of engineering.
I'm not acquainted with the Sigma EF610 but note that numerous after market Minolta/Sony compatible flashes aren't or are not totally. To be fully compatible it would need high speed sync, TTL and ADI compatibility (the latter conspicuously absent on some wannabees) and full wireless compatibility.
While you are picking up A57 bargains look for a Minolta 500mm f8 reflex lens (aka mirror lens). AFAIK this is the only AF mirror lens ever made. I bought one some years ago for $US200 and when Sony finally decided to stop making them they were about $US1200.