Originally Posted by
VitaliU
I would not recommend taking buses in Russia. Your best bet is a high speed Sapsan train (multiple daytime and evening departures, takes about 3 hours 40 minutes) or a sleeper train (slower, overnight. e.g. the Red Arrow train, takes about 8 hours). You can buy tickets on the Russian Railways website (it quite horrible but it works).
Many will probably disagree but I think Moscow is not an interesting city, give it 2 days and St Petersburg is very interesting and much more pleasant, 3-5 days should be enough to cover both the city itself and the country palaces and museums around it
I'll agree and disagree.
Taking the train is the best way between the two cities. VataliU has pointed out the two main options. The Sapsan is very comfortable and the night trains are good too (Generally, the lower the train number, the better it is).
The Russian Railways official site is the best. I wouldn't say it's that horrible either. Make sure you book before getting to Russia as trains do sell out regularly - especially around the weekend.
You might also consider flying, but I wouldn't recommend it as the airports are all so far from the centre.
Intercity buses = No, just don't!
In terms of the cities...
I always find Moscow to be very interesting. There so much to do and see there, it's a great place to visit.
I would say spending around 4-5 days per city would allow you to scratch the surface.