The AGA tripple T8 fixture is a great light, I now have it on my 90
gallon but I did use it for a little while on the 55 and loved it. The
twin T12 fixture instead uses a standard magnetic ballast that I have
now swapped with a 4xT8 electronic ballast and overdriving the tubes
at 2x. But the tripple tube is the best bet IMO, although the bulbs
that come with it are also very good, it allows you to mix and match 3
different bulbs to get the look you want. If I were to choose again
between 2x55W PC and the tripple tube, I'd go with the tripple for the
flexibility of being able to mix bulbs. Obviously if you are looking
to have a high light tank, you can pack more light in less space using
the PCs.

I think all the T8 fixtures from AGA use an electronic ballast. The
others are easy to tell because they have a starter behind the bulbs.

Hope that helps
Giancarlo Podio