The term 'T5' refers to the diameter of the fluorescent tubes which are 5/8 inches in diameter. T5s produce more light than T8/T12 tubes which are older in design.
T5 light being older technology, it is more stable and gives very predictable results. It has years of proven usage by expert aquarists as well as the horticulture industry.
There is a wide variety of bulbs across a spectrum of colours available on the market, so making adjustments to your light spectrum profile is easy. Efficacy is dependent on bulb quality and fixtures that have better built ballasts last longer. Reflector design can make a large difference in the actual amount of light shining into the tank. Hydroponic light fixtures that fit the tank size gives the most bang for the buck. Shop-lights fixtures can give good value as well.
The main advantage of LED over T5 lights is that it is more efficient, which will save cost (bulbs & power) in the long run. Designs are often more sleek.
Picking a commercial LED light for a planted aquarium that works well can be tricky. The technology is new and there are huge differences in quality & output between brands. Most cheaper fixtures on the market do not have an attractive spectrum selection and many are rather weak for growing plants. LEDs also do not produce UV light by default (unless UV diodes are used), while florescent lighting does.
It takes real depth of knowledge to know what to look out for in an LED fixture. Despite claims of "full-spectrum" or "plant specific" lighting - many fixtures on the market are still generic and while many do an okay job growing green plants, they are poor at stimulating pigmentation in red plants, especially when compared to higher end T5 specialty bulbs.
Many commercial fixtures will work okay, but finding a truly great one is difficult. I think that understanding PAR values and spectrum selection are the basics that one should know before making an LED purchase. There are a selection of LEDs that work better than the rest because of their focus on having more RGB rather than white LEDs, giving better color pigmentation and visual saturation than their peers, you can read about it in this article.
There is no physics difference in light being produced by an LED or T5 unit; however reflectors in different units can affect whether light beams are more collimated (better penetration in deep tanks) and of course, different units produces light of different spectrum/wavelengths. It is still generally easier to tune to get a good spectrum using T5 tubes as you can change the tubes to your liking. Many LEDs suffer from poor spectrum choice - this is not the fault of the technology itself but what aquarium light diodes manufacturers choose to use in constructing aquarium light units.
T5 lights have proven results plus there are many choices for bulbs. Many experienced aquarists still use them to great outcomes. Tom Barr's planted aquarium below shows this well with striking coloration produced through the usage of multi-coloured T5 tubes.
Below is Mark Crow's planted tank under twin BML LEDs. Finding a good spectrum selection can take effort though. Examples such as these fully dispel the notion that "LEDs can't grow aquarium plants" or that "LEDs can't grow red aquarium plants well".
At the end of the day, both can produce great planted tanks.
Head here to learn more about aquarium lighting for planted tanks
Head here to learn more about PAR values.
Head here to learn more about spectrum curves.