Both are important!
Depends on your tank size, what plants you want to grow, what inhabitants you have already etc etc....
I've been reading around on these forums a bit, and from the looks of it keeping plants growing happily and healthily is arguably harder than fish! Especially for the really high light plants!
Generally speaking you would want a daylight spectrum light source. IE the bulb or LED
or whatever you use has a colour temperature of 6500K or above.
Like I mentioned the wattage will depend on tank size and whether you're looking for low, low-mid, mid, mid-high, or high light...
(Watts per gallon) is a rule of thumb often used for the low/low-mid starting point.
But also worth considering is the light intensity... IE if you had a 50W T5 and a 50W T8, the T5 would be more intense as it's producing a the same amount of light as the T8 but from a smaller external surface area.
I'm no expert, I wouldn't even say I was semi-experienced. I'm a novice and what I've learnt so far comes from trawling these (and other) forums!
So in short, yes and no.