I have the DLC for the PC, and at startup, the game was different. I believe the DLC is active from the time that you buy it, there is no option to turn it "on" or "off", at least not from what I saw.
And in my opinion, the DLC makes the game significantly harder, but in a random way, if that makes any sense. It all depends on which levels you end up with -— if you end up with the regular Isaac levels (basement, caves, depths, womb), then the difficulty will be changed somewhat by the appearance of the newer enemies or bosses. So in that case, it will be ramped up a bit. However, if you end up entering one of the newer, "alternate" stages (cellar, catacombs, necropolis, utero), the random challenges and more difficult level layouts, paired with the possibility of two bosses in one level, as well as the more frequent new enemy/boss encounters, will make for an incredibly ramped up difficulty of play.
Hope my explanation helped, I for one have only played through the new DLC three times, and saw two seperate walkthroughts for it, so that's what I've gathered from what I've seen.
