I have both and really use only the transbrake.
IMHO, the line lock is better for your burnout then it is for the starting line. The linelock works by you pumping the brakes and energizing the linelock. You then release the brakes and fluid is reeased fromt he rear brakes, but kept on the front ones until you release it. Again, good for burnouts. To install, you just splice the linelock into the front brake line.
The transbrake is installed inside your transmission. That is why it costs more. You also get what you pay for with them. The cheaper ones have a much slower release rate. Do a search on transbrakes, because we had a thread not to long ago that told how they work. Combine it with a "two step" and you have a great combo. You can stage, set the brake, floor the car and it will stay at your selected rpm until released. When leaving the line, it brings back memories of the days we help mom's car at 5 grand and dropped it into drive!
My friend, who is a hard core bracket racer, using both on the starting line. When he lines up, he pumps the brakes and sets the transbrake and linelock at the same time.(Same button). The linelock stops the little bit of creep you get when setting the transbrake.
Hope some of the helps.
1992 Firebird 355/Six Speed (Soon to be LT1)
1991 RS 350 / 700-R4
1987 Toyota Pickup 383 / 500 + HP 10.963 @ 119.95 Slicks / 11.997 @ 114.23 Radials