Not going to lie, I had to lookup the MR20 lol. Didn't even know it existed.
Let's see...easy way to explain it is with a balloon analogy I guess.
When it comes to gain with omni-directional antenna (which both models are using), think of a filled up balloon being gain 0 (isotropic antenna).
When you place the balloon on a table, and place your hand of top, start to push down on it. The balloon will start to 'push outwards' and the 'signal' goes more along the horizontal plane (left/right) vs the vertical plane (up/down). The more you push down on that balloon, the more gain you have.
The gain being 'higher' on the MR20 will in theory provide more RF on the horizontal plane, so in theory if you had an open field miles across, and you placed both access points there, you could potentially walk further away from the MR20 and maintain better signal (assuming they are both at the same power).
However in your situation, the gain difference is going to be negligible, especially for a typical office environment.
Comparing the specs, they both appear to be very similar (2x2:2), but the MR33 has a 3rd radio and bluetooth radio as well. I would imagine the MR33 has better hardware inside (CPU/RAM etc.) that would also help it perform better under load, but that is speculation on my part.
Here below is their antenna patterns, and they both seems very similar which is good.