に気をつける is a much more popular expression, from what I see, compared to を気をつける. But the latter is still not that rare.
Is there any significant difference between these two and something to be wary of upon choosing to use either?
を気をつける is much rarer. The BCCWJ gives the following numbers:
に気を（つ・付） 1363 results には気を（つ・付） 181 results は気を（つ・付） 155 results を気を（つ・付） 26 results
に（は） 89.5% は 9.0% を 1.5%
First of all, I think that this shows that に or は are much more common. Moreover, I think that in most of the cases, you can replace を by に without much trouble.
However, in examples such as
に may be misinterpreted as a location marker, because the preceding word can also describe a location, even if it used differently in these sentences. (The に in ～に気をつける is rather the "direction" marker (used in a figurative sense).)
Also, を can be used as a last resort, because に（は） has been used already, as in