i think you miss the point of derma rolling,
its not about making yourself bleed,
you should use it kinda gently, just to stimulate the blood beneath the skin,
than you should rub pure aloe vera on it to help with the healing process,
than this healing process will renew your skin on the long run, so that might be benefitial,
after about 2 weeks of regualr ED or EOD or MTS use your skin should start to peel, than it starts to renew,
like snakes skin.
anyway - even the 0.5mm wont go all the way in, so there is no difference,
and besides - 2mm is huge and not suggested by the derma rollers companies to use on scalp or face (since the skin there is very thin, its more for leg stretch mark ETC )
i used the 0.5
than i got 1.5, and its a bit stronger if your press harder, but if you do it right you should be good.
i got it basically just because i wanted to grow my hair and the 0.5 wouldnt reach the scalp.