Mercury vapor in the tube creates UV light, which strikes the phosphor coating on the inside of the tube. The phosphor absorbes the UV and converts the energy in the UV into the visible light that you see.
Over time, the mercury vapor is absorbed by the coating, and the lamp will loose much of it's brightness.
Some tubes have a protective layer on the coating that will prevent the mercury absorbtion. These lamps will loose only a small amount of their original brightness over time.
For growing plants there are two important factors: How much light is produced (lumen) and how efficient is that light when it is used by the plants:
Brand Type Efficiency
Philips TL D 927 240
Philips TL D 930 230
Philips TL D 940 220
Philips TL D 950 220
Philips TL D 830 195
Philips TL D 840 195
Osram 21 195
Osram 31 195
Sylvania CWX 184 195
Sylvania WWX 183 195
Philips TL D 827 180
Osram 41 180
Philips TL D 865 170
Philips TL D 33 160
Osram 20 160
Sylvania CW 160
Philips TL D 54 140
Osram 10 140
Sylvania D 140
Sylvania CWX 135
Duro Test True lite powertwist 135
Sylvania IF 130
Sylvania WWX 130
Duro Test True lite 130
Osram 19 120
Osram flora 110
Sylvania Grolux 100
Sorry for the layout.
From this table you can see that Philips 927 is 2.4 times as efficient as Sylvania grolux. So you can use either 100 Watt of Philips, or 240 Watt of grolux to get the same production of plant mass.
In Holland the Philips 830 and 840 tubes are very popular for aquarium use. These tubes also have the protective coating to avoid mercury vapor loss, so they do not loose much of their initial brightness. Use them until they are dead. The same goes for the 9xx series in T5 from Philips, which give even better color rendering. The 830 and 840 types are very affordable, only a fraction of what you may have to shell out for a special 'aquarium' lamp (that you have to replace after a year or so).