They're both private cryptocurrencies so you can't go wrong with either. I guess from a investment standpoint, Monero would be a better bet.
Why do you believe Monero is better from an investment standpoint? Is it because it is top 10 market cap or the fact it has a bigger community?
Monero is from far the best altcoin when it comes to security measures.
I have invested in multiple coins and Monero haven't give me a big profit, and about DeepOnion
is the first time I had real interest on it so now I'm checking the official bitcointalk DeepOnion thread.