You can argue that online poker is tougher to beat than live poker. This is because online players are a lot more technical. Many are used to multi-tabling and fast decisions. They play way more hands per hour and have a lot more “experience” of scenarios and hands.
Plus, online players have a lot of tools and resources they can use for online play. So, the competition is certainly tougher. But playing online will certainly improve your poker live game – if you make it there, you make it anywhere.