They are both a double edged sword.

I'm more of an idealist than anything, but even I can get cynical every now and then. In my life i try to keep a balance in between the two, but leaning more towards idealism; I acknowledge the bad things that are happening in my life and try to tackle them as best as I can, keeping an optimistic outlook on the result of my efforts.

If you think that everything is fine and dandy and refuse to acknowledge your failures or the bad things that are happening in your life, that's not good, because that's denial and you are unable to improve off of those failures. If you are completely focused on your failures and refuse to look past them, then that's not good either because ultimately you're not striving to improve off of your failures, and thus you never will, which means you're going to have the same failures picking at your mind for a very long time.

Aanalogy; Let's say that you're an artist. You paint a picture. The picture has a lot of flaws, but then again, it has a lot of positives going for it. Pure idealists will distract themselves from the flaws in the pictures, the things they could improve on, and only look at the positives. That's not going to help. Cynics look at all of the flaws and refuse to look at the positives of the picture, only focusing on the flaws, thinking that 'they'll never improve, the flaws are too prevalent, they have no talent as an artist and they're never going to be good ever. why even bother trying'. That's not going to help.

TL;DR; pure cynicism is bad. pure idealism is bad. a balance between the two is good.