One of my favorite ways to use an aqua pen is with a heat embossed line art image. Wet the area you want to color in slightly with the aqua painter then dip the a.p. in ink and lightly touch it to the image. The color will spread within the image beautifully. A simplified version of this technique is shared here
The blender pen can be used to pick up ink either from the lid of an ink pad or from a dye-based marker and then color in your image. It's also a good tool for REMOVING ink from a stamp. So, for example, you could ink up a stamp, run the blender pen across an area to remove the ink*, and then add a different color ink with a marker.
For both of these products, you'll want to try them with the different types of paper you have. The aqua painter is like a paint brush, so watercolor paper is likely best. The blender pen works well on Stampin' Up! Shimmery White card stock and can be used a little on their regular cardstock. Often it can cause the standard Whisper White to pill a bit.
If you do a search in YouTube, you'll find a LOT of videos comparing the two, sharing techniques for each, etc.
*just scribble the Blender Pen across scrap paper to remove that color from your pen