Getting HAMMERED

The ideal hammer for an individual is determined by head proportions, shape, weight, and handle length, grip profile and diameter. 

David Robertson for example uses a standard hammer that I personally find very uncomfortable to use. We both have considerable experience in equipping many students with tools over many years (I must easily have 50 hammers here in a dozen styles). I should also say my own personal hammer has been in my hands since the early 1980's. At 800 gms, I do about 85 % of my own work with that one hammer. (Any smith is likely to say the same thing about 'their' hammer!) 
Raw weight also needs to be considered against stock size being formed - and the desired effect for that forming. Generally heavy means a harder impact, but also means less control. Control is significantly more important (in my view) than mere power. This also can become a significant safety problem. Use of too heavy a tool for an individual, especially if the grip is not correct, can easily result in tramatic damage to elbow tendons, development of joint damages. (Yes - this specifically known from painful personal experience!) Tendon damage is a life long injury that never fully recovers.

1) Princess Auto / Deals / KW Surplus 'American' style square head cross peen, made in China = 800 gms / cost about $10 each.
Note that these now have very bad plastic handles which need be replaced (see below). Over the years the handles have gone from junk wood to fiberglass to the current (horrible) plastic

2) Princess Auto Ball peen, made in Italy = 710 gms / cost $23 each
This is an excellent hammer, which outperforms anything else in its class. Great shaped ash handles. (There is a 900 gm version at $32 - also an excellent 'starting' hammer)

3) Professional Farriers Supply (Orangeville) Rounding hammer = 800 gms / cost about $30 each
This is notably a very short handle, and an extremely balanced head. (So ideal for physically smaller / less strong individuals). (There is a 1000 gm version at about $45, which again has the balanced head) Rounding hammers (on flat, one slightly crowned face) are excellent for a number of forging tasks btw.

Handles - Home Hardware 16 inch ash blacksmith's handle / cost about $8 each These are excellent handles. You will have to ask to have them ordered in (these are open stock, but most stores just don't carry them).

The image here shows the three hammers listed:
 - The American pattern has a fiberglass handle - these hammers were available this way for a short time a number of years back.
- The hammers have wooden handles conform to what I personally consider the correct profile. This thins the handle just below the eye to reduce impact shock. (This does limit the life of the handle, but handles are cheap and elbows are not.)
- My own hammers (green tape) have also got extra (hockey & foil) tape protection just below the head. Also silicone rubber tape on the grip (again reduced impact shock).

As you can see - I consider hammer selection critically important to effective forge technique - and long term working ability.