Step-up or step-down ring adapters?

Am new in photography and i need some help. I have a nikon d5000 with an 18-50mm lens (52mm ring), and 70-300mm lens (62mm ring). I wanted to buy filters that i can interchangeably use with both. What if just buy a one size filter so i can use it both on the 52mm and 62mm rings (as above) and use a stepping ring adapter? Would i buy a 52-62mm step-up ring adapter and 62mm filters? Or a 62-52mm step-down ring adapter and a 52mm filters?

You would want to get filters that fit on the larger diameter (62mm) lens and then a step up ring to use them on the smaller lens. If you try to use 52mm filters on the 62mm lens all your photos taken with the 70-300 will have a dark circle around the edges from the filter ring.