I'd go with the longer option. Shorter needles seem to come with a higher chance of reactions at the site.
I've been doing reta injections twice a week for two months and it still feels just as hard. My husband has been doing them for me since I'm too nervous to do it myself. I bought a pen designed for hidden needles and now I'm looking at 33-gauge 4 mm tips but wondering if 6 mm would be better instead. I'm carrying extra weight so there is plenty of fat to work with. Will the shorter ones still reach the right layer when the cap is on?