pro's and con's either way...

open source mean anyone can find and patch the bugs, but also anyone can find and exploit bugs.

I feel that often open source is more secure simple because patches usually come out faster if a bug is found. (but this is not always the case.)

at the end of the day security is more about you and your habits then what OS you are using.

don't open ports you don't need.
don't run services you don't need.
keep passwords strong.
do patching often.