You produce sound by flowing air through your vocal folds and making them oscillate. More oscillations produce a higher sound frequency, while less oscillations produce a lower sound frequency.
You can also alter the shape of your
mouth to produce different sounds and words.
The above taken into consideration, I'd choose words that cost the least amount of effort to produce.
Low frequencies put less strain (less oscillations) on the folds, however you might need more air flow (lung pressure) to create the volume required for the speech recognition.
Sounds: As far as the altering of the shape of your mouth and vowels goes, I think eventually a certain type of repetitive strain injury could occur depending on often you use the words.
Also consider this: if your mouth is closed at rest, try to pronounce: "up". Now try the same starting with your mouth opened. In the former case you first need to open your mouth, while in the latter you can start by flowing air.
Look closely at the amount of effort you have to put into producing a certain word. Depending on your 'natural voice' (having a certain frequency), your lung capacity and your resting position you should decide what feels most comfortable.
Regarding "touch" and "click" -
The 'ou' in
touch tends to have lower frequencies than the 'ick' in
click. Also to connect the 'c' to the 'l' in
click you have to move the tip of your tongue to touch right behind your teeth in your upper jaw, whereas this is already the starting position for the 't' in