What are the benfits of a premium account

There are trade offs with a premium tank so the decision to buy one is not as simple as saying 'does it make more money?'

Firstly a premium tank is costly to purchase, the high tier ones with the best earning potential are 7500 to 12000 gold (£30-40) each and there's no point in buying a premium tank without spending another 1000 gold on a 100% crew, you will also need to spend another million silver (2500 gold) to get Ventilation and a Rammer. So that's a layout of approximately £50-60.

Secondly you need to know that the premium tanks are not the best tanks (apart from the Type 59 of course). An equivalent tank of the same tier will generally out perform a premium tank by a small but appreciable margin.

Thirdly, for grinding out silver a premium account effectively doubles your income. Although it only boosts your basic income by 50%, that is before you pay for repairs and ammo which can easily halve your final income, but with premium that 50% bonus to the gross amount works out as about 100% on the net profit. Premium tanks only have a significantly reduced repair cost which is why they're good for grinding silver.

Fourthly you need to be at least an average to good player to really benefit from a premium tank, just turning up and going AFK in a premium tank is only minimally more effective than doing the same in any other tank. You quality of play massively influences your income.

So yes, premium tanks can earn more money, but it's no as simple as buy premium > profit!

EDIT: I quickly found this site after finishing this post; it contains averaged data for a large number of tanks across some 940,000 battles, including WN7 and PA Efficiency stats.

The various rating systems out there (WN7 is the one I prefer, and is the one Radium mentioned) can in fact be used. While Xenox accurately pointed out that such equations compare players to other players, data subsets could be substituted for full player stats to give ratings for that player in a given tank. (See footnote.) Averaging such data across a large number of players ought to give a moderately accurate metric by which tanks could be compared - a better tank would, on average, yield higher WN7 scores.

Unfortunately this would be by no means a perfect system - tank popularity could skew data, and it is difficult to account for player skill or tank crew levels - but it ought to give a decent impression. An additional difficulty lies in acquiring this data from a user level - there are a few tools that enhance the official statistics pages, but to my knowledge acquiring such data for individual tanks can only be done in-game, and the in-game service record lacks important data used in WN7 calculations.

As an example, I used the WN7 formula to score my BDR G1 B Tier V Heavy tank, and received a score of ~1400. I had to fudge a couple numbers (I couldn't find spot or defense point scores specific to my BDR tank) and there are likely rounding errors, but it can be done. With a little encouragement, the WoT community may even be convinced to write a tool to acquire and compile the relevant information and produce scores for specific tanks! I would be interested to browse such a tool myself...

Footnote: Minor modifications to the formula may need to be made to account for smaller numbers, particularly in the Number of Battles Played - alternatively, that segment could be left out entirely. It didn't come up during my calculations; using only stats from Tier V or higher tanks, the term will always result in 0. Comparing lower-tier tanks may be difficult and the term would likely need to account for some full-player data to avoid skew from experienced players going sealclubbing... but I'll leave such discussion to a larger and more math-experienced crowd.