I don't understand the physics of the lights, but I have used both to grow, and the T5's, especially if you get the high output, are much brighter. The temperature around the top of the fixture is about 95 degrees, so you may want to put it in a cool basement type area and run a fan to deal with the heat. I've never tried seedlings under mine, though, but it's doing great keeping tropical waterlilies growing over the winter.

I have used cheap T8 shop lights and they just don't hold up over time. I will only buy good T5 fixtures from now on, to replace the T8's as they die.

Of course, the T8's use more power, so they are more expensive to operate, and they are more expensive to buy as well. But if you want bright light, they are your best choice.

You can get T5's online in various combinations of lengths and number of tubes, you shouldn't have any problem getting a 4 bulb 4 foot.

Are you going to be using it year-round, or just for a brief period each year to start seeds?