Buy Arista 400 film from Freestyle. Repacked Ilford HP5 but less expensive I believe. No use running more expensive film. Also There is no difference in the shutter settings on a Holga, I don't know about a Diana.

As already pointed out, you best control is through processing of the negatives. I usually play around with HC110 in various dilutions to get desired results. You will get the most consistant results shooting on sunny or light overcast days.

Another option which I do with Holgas is to have two or three cameras. You can drill out the aperture on one, test focus range and use that camera on more over cast days. This does tend to soften the image slightly more but still maintains the sharp middle area that holgas are known for.