I cannot tell you whether Megatlon is WORTH it because I have never been a member, but in my gym search I checked them out. I would say it depends on what you want and need. They seem to have much higher quality MACHINES. But I was really turned off by all their extra fees - i.e. the locker use fee, the required contract - something none of the other gyms in my neighborhood require, a cost 2 or 3 times regular gyms, and the sales pitch pressure from the 21 year old guy who showed me around.
But if you want to use modern fitness machines beyond basic weight training equipment, it may be your best option because they seem to have PLENTY of good machines at their locations compared to our other local gyms which seem to have few machines and get really really crowded in peak times.
Megatlon has nicer facilities, but I found that a nicer looking gym doesn't get me out the door working out. I need a class, accountability. At my local gym I can go to unlimited aerobics classes which pack weight training, stretching, cardio, and totally kick my butt in under an hour for 70 pesos a month! I had a lot of bias against aerobic classes..it seemed so 80s and silly, but I have really enjoyed it and the teachers have been great! For an extra twenty I can either also do Pilates - or use the weight room. I also started with just Pilates, and I, too, needed something more intense.
Why not try a no-commitment gym for a couple months and see if that steps it up for you? If not, you could always switch later to the pricier Megatlon. Take advantage of that great thing most Argentine gyms offer which no US gyms have - NO contract!