From my personal experience i would say codechef is very friendly for beginners. Codeforces questions are tough compared to codechef’s. And because you have limited time, you should practice and compete in codechef long challenge every month.
Codechef long takes a lot of time but is very good for learning new stuff, codechef lunchtime and cook-off happen once a month only, for 2-3 hrs so you should definitely go for those,followed by upsolving.
But codeforces contests are regular, on an average have 2-3 days of cooldown between consecutive contests, so for regular practice go for those.
Apart from Codechef long challenges, I think both are doable simultaneously due to scarce rated contests on codechef compared to codeforces. Rest is up to you to choose.