I have only limited experienced of visiting Russia but I would go for St Petersburg - relatively easy to converse in English (eg lots of signs are in English). St Petersburg residents seem to have a much more Northern European attitude rather than someone brought up in the harshness of Moscow (but I haven't been to Moscow so someone might correct me). I would expect the girls in St Petersburg to be more reliable. Hotels and flights are also cheap.
People's attitude is similar in Moscow and Saint Petersburg.
There are way more flights to Moscow.
The choice of WGs is A LOT better in Moscow. Check intimcity.nl
Guess most of them will be able to do basic conversation via WhatsApp.
The airport in SPb is a bit closer to the city centre, but the public transport is a bit fucked up, but still usable (I would not recommend using a taxi).
Everything is roughly 1.5x more expensive in Moscow (hotels, girls, meals).
Nearly all of the signs are repeated in English in both of the cities. Both are quite safe (just exercise the common sense) but Moscow is a lot nicer and cleaner.