Choose your spot based on what matters most to you at the concert. If you're a fan of the band and want to feel the energy, music, and atmosphere as much as possible, then the floor or front rows of the stands are your thing. You'll be closer to the stage, see the artists better, and hear the live sound quality. The downside is that people usually stand there, it can get crowded, and if you're not the tallest, someone might block your view. At Korol i shut concerts, the crowd is usually pretty active, so keep that in mind.
The middle floor or first balcony is a good middle ground. From there you can see the whole stage well, the sound is good quality, you're standing in relative comfort, and you don't risk being crushed by the crowd. I'd recommend this option if you're not ready for the extreme of the floor. I'd only suggest the upper balconies if price is critical for you - you can see the stage from there, but the artists look really tiny, and the acoustics are already worse.
One more tip: check out the seating charts for Olimpiyskiy online, read reviews from people about visibility from specific seats. On fan forums, people often share their impressions about where you can see best in this particular venue. That'll help you make a decision without any disappointments.