Do you know how well having two systems works?
Works fine. I haven’t done it on a chromebook myself, but it’s nothing special. Google it. In fact, I just found an article about installing ubuntu on top of chrome OS —that is, using the same kernel; which is entirely possible since chrome OS is a version of linux anyway— that looks pretty interesting. But you could also do a plain ol’ dual-boot setup, or replace chrome OS completely. None of these things are difficult to do.
No matter what you choose, make sure everything is backed up before you start.
and is it vulnerable to security issues?
…no more so than normal? Quite a bit less than Windows. Probably safer (broadly speaking) than OSX. Who knows? being designed for “everyday users” and shipping as the default OS for a mainstream device, chrome OS (or Android) may well have some vulnerability that a more “nerdy” Linux flavor doesn’t.
Is there something specific that you’re worried about?
do you prefer ubuntu or mint?
I use Mint, but they’re basically the same “under the hood.” From a UI perspective, Mint looks more like a traditional, windows-ish PC desktop, while Ubuntu has a look somewhere between OSX and Android… kinda… if you enjoy learning a “new look,” try Ubuntu. It probably has more community support as well.