I feel the effects of alcohol very quickly. I need much less than
other people in order to get the same level of enjoyment.

My question is this: are negative effects on the brain relative to the
level of intoxication experienced or the quantity of the substance
consumed? In other words, is it safer for me to consume these
substances coz I don't need as much? What about affects on the liver
and other negative associations?

Is the situation different for marijuana?

I've also heard that people who need less of a drug are less likely to
become addicted. Is this just because they are less likely to take the
quantity that creates physical dependence?

YES, you have heard right.

Some of us have a liver that metabolizes ethanol (drinking alcohol)
differently.  To my knowledge, the EXACT biochemical means by which
this is accomplished is unknown --- but this observation has been
repeated again and again and again.  In my own case, beer gives me a
feeling of euphoria that is more intense and long lasting than grain
alcohol or hard liquor.

Also, if alcoholism runs in your family . . . you are at increased
risk of becomming an alcoholic.  Even if there was no alcohol used in
your home while growing up, if one of your first degree relatives is
an alcoholic, you are at increased risk.

And, (on good authority), you ARE at increased risk of becomming and
alcoholic if ethanol gives you more of a "buzz" at low doses.  To
actually BECOME an alcoholic, you would have to make drinking a
regular (rather than occasional) part of your lifestyle.  But the risk
is there.  So be careful.  Always choose to drink in moderation. 
Always choose to drink at a place and time where you KNOW that you can
remain in control of yourself and your situation.

THAT last is good advice for anyone!

I agree..That's good advice to take to heart..I know because I am in
recovery myself and had to learn this the hard way..It is very easy
crossing the line into addiction. Here's how I finally got clean:
http://www.drugrehabcenter.com

As far as "addiction" goes, "addiction" really isn't a medical
diagnosis. It is a popular concept more than a medical term.

As far as marijuana's effects on the liver and brain, you could
consume large quantities with no real fear of damage. According to
which US Federal Government authority you want to believe, the lethal
dose of marijuana is either one-third your body weight (US National
Commission on Marihuana and Drug Abuse, 1973) or 1,500 pounds (Ruling
of Chief Administrative Law Judge for the DEA, 1989). Both reports are
available at http://www.druglibrary.org/schaffer under Major Studies
of Drugs and Drug Policy.