I'm also taking a weekend (3-day) trip to Russia, and trying to decide between Moscow & Saint Petersburg. However, I'm not interested in the shopping or dancing, rather I'll be there to expand my photography portfolio. It sounds like St. Petersburg will be more attractive, though I'm interested in more than just "pretty". I'm also seeking shots of architecture, cultural differences, bizarre or traditional clothing styles. Any advice on which city might be best for this?
Architecture is great in both cities. In Moscow there are tons and tons of old churches and buildings that were rebuilt after the fire of 1812. Petersburg has great architecture as well but comparatively Moscow is larger and 600 years older. Also remember that Petersburg was a planned city, built by Peter to be the capital so a lot of the architecture there is monumental.
I doubt that you will find too many cultural differences and certainly no really bizarre or traditional cothing styles, other than that most Russian men wear black.
Personally I prefer Moscow but I would check to see which is easier and cheaper to reach.
Ruth

Ruth- Thank you so very much for your insights. I'm leaning toward Moscow, but have a few concerns. Will I have trouble taking photographs? I'm most interested in photographing outdoors, so if I can't take photos inside a museum, etc. that's no problem. But I'd hate to go and find out I can't take photos of the Kremlin, Red Square, and St. Basil's Cathedral. One guidebook I have says you have to leave your bags before entering the Kremlin, so I'm concerned I won't be able to do any photography.
Thanks in advance for any insights!
The only place where people report having problems taking pictures is occasionally the Moscow metro. That said, it is easier in Moscow than in Petersburg where the prohibition on photos in the metro is more strictly enforced.
Kremlin, Red Square and St. Basil's are all places where photos can be taken. Actually there are few places that are restricted.
You must leave you luggage and backpack but you can take a photography bag if you buy a special permit. At least this is what I have done in the past. I haven't been to the Kremlin in almost 5 years now.
Actually some museums allow photos inside with the purchase of a permit.
Ruth