According to the manual, there is no difference.
A and RB buttons are both mapped to the handbrake, so it's just for convenience.
Personally, I alternate between the two based on what I am doing.
Example:
When I want to change weapons, X, and make an immediate sharp turn, I would use A.
Otherwise, I would use RB (especially if I had to answer the phone).
Hope this helps!
The Jewel Store Job
The maximum take for the mission is $4,946,153 if the player smashes all 20 displays and chooses a good gunman. $2,500,000 will then go to Martin Madrazo while the rest gets split between the crew.
The Merryweather Heist
No payoff - Reason is spoiler
The Paleto Score
The crew manage to get around $8,016,020 during this heist.
The Buraeu Raid
no payoff
The Big Score
Choosing the obvious approach will yield around $31,000,000 per character, as long as you choose a good driver to pilot the helicopter, otherwise half the score will be lost. Taking this approach is far more difficult.
Choosing the subtle approach will yield around $24,000,000 per character. It is important to have a good hacker; lesser skilled hackers will cause lag in the traffic lights making this part of the heist more difficult.
Good Recommendation
solarosh
On a side note, I recommended only doing the first Lester mission for now. Reason being, these missions biggest payoff is in taking advantage of your advance knowledge of the repercussions these missions have on the stock market. Doing so is some of the best money in the game, and logic dictates that the more cash you have on hand to invest before rigging the market, the higher the dividends you will reap... Remember -- Always Buy low, Sell high
Sources
http://gta.wikia.com/Heists
http://www.ign.com/wikis/gta-5/Blitz_Play (solarosh recommendation)
Best Answer
Generally, the Metallic paints have more of a sheen to them and are somewhat reflective, classic paints are glossy but generally aren't reflective. Classic paint is most similar to cars in real life, and is the default color type for the vehicles NPCs drive.
There's a similar discussion about this on Reddit too.
Here's an image to demonstrate how reflective metallic paint is:
And here's a car with a classic paint applied. You can see that this is still kinda glossy, but comparably flat to metallic paint:
Even now, these images don't quite display the difference fully. The qualities of the two paints is even easier to recognize in-game. If you have the money to spare, purchase paint in the game and compare it to see for yourself how the two coats differ.