The dual-gang type is useful because you can hook them up to obtain different range of values, making it useful to dial various bands.
The air capacitor is easy to work with. Easy to fine-adjust. It occupies larger space. The smaller miniature type is convenient for pocket transistor radios, and may require more work on your part to fit a tuning knob properly.
Or you might like to try an inexpensive homebrew. Check the internet for articles on making an air capacitor from sheet metal and wood.
After reading one article I tried a fun experiment. I cut two pieces of aluminum foil, each several square inches, and rolled them between sheets of paper. Clipping my jumper wires to the aluminum created a capacitor. Value about 1 nF, which can be adjusted somewhat by squeezing and relaxing pressure. Cheap and educational.