First; Why are you purchasing the paint and stain? Normally, at least for us, we purchase all materials needed to do a job. Maybe your situation is different.

You need to stay away from Home Depot, unless you want to re-paint/stain sooner. Go to a real local paint store. The type of stain depends on what your after, the final look. You have a choice of semi-transparent, semi-solid and solid color. Basically, solid looks like paint and last the longest while semi-transparent looks like a stain but needs redoing much sooner.

Benjamin Moore make really good exterior stains and paint. You can also go to a Sherwin Williams store. Cost effectiveness depends on quality and how soon do you want to restain/paint. Buy the best you can afford.