Both... actually. If you put high gain tubes in the normal gain stages (usually V1 & V2 or V1 & V3 on Marshall TSLs) it will boost your gain in all channels, if you put it in the gain channel gain stage (yes I know) you'll get more gain in this/those channels.
If you are into metal you are friend of mine, whimps and posers .... wait what was I going to say ..... Oh right, if you are into metal and you play lead very good idea to get a balanced one for phase inverter, not sure how it works with other amps but in my AOR its V3, I also got a gold pin for my V1 to have a rich sonic complexity and my V2 and V4 are high gains for "balls to the wall" purposes.
You should also remember that the tube you put in the first gain stage will affect how noisy your amp is... So you should put the quietest tube you can find in there.
A lot of folks (myself included) find the balanced triode thing to be so much snake oil. You might find this thread an interesting read. A number of the posters there are amp builders, amp techs, and one guy is a tube guru at Groove Tubes. It also has some discussion about the effects of swapping tubes in different positions.