I just ordered the KM 28-75 f/2.8 for $355 from digitalfotoclub.com. It was $369 at keh.com (the second least expensive place I found it). This lens has received rave reviews everywhere I've read. The Tamron version also has great reviews and may go for $25 less (or more). I'm very excited to get this lens! For now, I think I'll also get the classic Minolta 70-210 f/4, as it can be purchased for less than $200 on the used market and reviews say it has great optics, even though it's a bit of a tank! Someday, I'd like to get something that continues with f/2.8 through 200, but getting a great lens at f/4 for $150 or so versus a super lens for $800+ is a compromise I'm willing to make right now!
Anyway, I wasn't sure if you had considered the KM 28-75 f/2.8, or if you had and were trying to keep costs even lower, but I thought I'd throw it out there. I haven't done a lot of research on Sigma lenses in general, and not the ones you mentioned in particular, so I'm sorry I can't help you there!
I posted this in another thread, but just incase you didn't see it, here it is again...
I'm not sure about "professional" reviews, but I enjoyed reading reviews at these sites:
photographyreivew.com
dyxum.com
photozone.de
http://www.photo.net/equipment/minolta/lenses
I also put out a couple inquiries at dpreview, as well as searched their archives, and I received/found good info.
Happy hunting!
Elaine