Moneywise it is less expensive to get the power rating first, then the
glider. However if you are under 16, then glider may be a better option
to do first. The glider rating is fairly easy once you have the power
rating. All those tows cost quite a bit. You will already know how to
land a glider if you can land an airplane (well, it may take two or
three practice trys to do it well). But essentially yoiu are there.
Experienced glider pilots can fly for several hours on one tow, so once
you have the glider rating, it is less to fly gliders, from what I have
seen. I do think all airplane pilots benefit from some glider training.
If nothing else, you really begin to believe in landing with no engine!