We had some forum problems which should now solved after we upgraded the WowBB Software earlier this weekend. So, you may not have seen my previous posts, right before the forums were taken down after we noticed the problems (since they're in the wrong order within this thread).
Anyway, the Sigma would be twice as bright. So, I'd personally go that route instead. It looks like B&H has another one in right now, according to the listings in their used department. So, you may want to see if they'll just swap out the one you got for it (giving it a try in the store if you're planning on another visit anyway).
I skipped the wider primes, starting out with my Minolta 28mm f/2 as my widest prime, since my Tamron 20-40mm f/2.7-3.5 covers the 20 and 24mm focal lengths, and it's sharper compared to both the Minolta 20mm f/2.8 and 24mm f/2.8 primes, even at f/2.8 (according to the MTF tests at photodo.com).
I wouldn't mind having something a stop brighter than my Tamron 20-40mm (like that Sigma 20mm f/1.8 ). But, I don't have a need for the Minolta 20mm or 24mm f/2.8 primes (since I've got that end covered with a bright wide zoom that should be sharper compared to these Minolta primes).