Oh dear god, I hope I'm assuming incorrectly, that this and that .22 listed are being considered for self defense?! If you're looking for a novelty to own for the sake of owning, sure, why not, but if you're genuinely considering trusting your life to one, I hope for your sake the need to use one never comes. Size of the shot isn't an end-all, admittedly coming from someone who favors a .45 for a carry, but it IS a factor. .380 or smaller is generally considered too small to rely on for self defense (Yes, they CAN kill you, but are they likely to stop an assailant set on taking your life? No.) as is round count. There are people (NOT hopped up on drugs) who have continued moving with 5+ rounds of .45 in them. They didn't survive, no, but they kept charging the person they were attacking and even got a few hits in. When you're defending yourself, unlike hunting, a lethal shot isn't enough to stop the attack. You need a shot that will physically prevent them from being able to move, INSTANTLY, and in a high-stress situation like self defense, the best way to achieve such a shot is accuracy by volume. I'm not a professional instructor by any means, so take my advice as you will, but a derringer is NOT a self defense weapon. I honestly think, as much as I love carrying my 1911, it's round count is almost too low. If you want something small that could be used for self defense, look into the Sig 938:
http://www.sigsauer.com/CatalogProductList/pistols-p938.aspx
But honestly, you would likely be better served by a Glock 26:
http://us.glock.com/products/model/g26gen4
Again, that is all under the possible incorrect assumption you're looking for self defense. If you just want a range toy, go for it. But it WILL be unpleasant to shoot.