Also to add, effective means it works and will do what you want. If u do the square properly, regardless of using the short version or the longer version of the mantra, the working is doing its job just the same. Power adds to how quickly and easily the working will manifest. The squares themselves are very powerful so even if one's a beginner or not as adept as others the square itself is designed in a way to still manifest regardless. On the downside, the squares are very specific in how they must be done which means if you mess up once then the square is ruined.
If you think you are going to do the entire square without any issues then the long mantra should be used. If you are still new or a beginner in magick or believe that at some point during the square there will be a day in which you know you will be short on time then use the short mantra.