I used to work with rented apartments in St. Petersburg a few years ago intensively when this option offered tourists savings of about 50% in comparison to hotel prices. But within last 6 years about 50 new so called "mini-hotels" with 4-12 rooms have been opened right in the city and particularly in the city center. This has changed the market so that the prices evened out, but mini-hotels win based on the quality of the service they offer. For the same price one gets in mini-hotel:
So – my recommendation is to go for a mini-hotel instead of an apartment.