I have a a little giant 100lb and Nazel 3B, I'm a big fan of powerhammers the bigger the better I work everything from 1/4" stock to 4" solid square with it. The self contained I find are much nicer for long controled tapers etc they have real nice rythmn to them.
I use my little giant more for one hit work, I have real good brake on it and it can hit nice and hard single blows. I also use it when doing work were I need to have a certian die for one procedure and jump back andf forth from one hammer to the other.
Both machines I run radius flat dies and have built some different profiled dies that drop on the bottom die, They are the same size and have a flat bar band on them with a set screws to hold them on. Either that or I have one those drop on tool holders.
For the flat dies I found that an 1/8" radius isn't quite enough for me, I have the very edge of the die about 3/16 round but crown it in just slightly, about a 1/2" or so in which helps keep the straight die marks off the work when smoothing it out. These are on dies that are about 4" wide and 8" long so the amount you'd want on smaller dies would take some experimenting.
I highly recomend the Clifton Ralph tapes and the last Dave Manzer Video had some good demo's on tooling.
Spending time with someone who knows how to run a hammer is a must,
I had a couple years experience on hammers all self taught but had only seen someone else use them on videos' and once demo'd at an NWBA conference, I was a bit cautious using the Nazel to its full potenial so I went and spent a weekend with Terry Carson (president of the NWBA) at his place and had great time, and reassured me I was going in the right directions with my techniques and gave me confidence, well worth the time and money.